On the second print, try just a small amount of extra glue around the print area, see if the glue gives a little bit of "extra grab" to the filament. Works for me sometimes, I think I have experienced this same issue you are having.
I find that the glass plate feels weird where a print has just been removed...almost a little too dry. Dunno if this makes sense lol :???:
Well it does make sense i guess but in that case why does it work right after without changing anything?
The filament is extruded correctly before starting to print by the way so it should not be a problem with extrusion
This happened to me too. Maybe the filament was ground down and while retracting at the end of the failed print, it went over the problematic filament part and straightened it a bit. Then for the next print, the knurled wheel could grab the filament properly and extrude.
Yes could be... should i try pushing the filament by hand in that case?
Just look at the extruder and see if the filament is ground down and the knurled bolt turns without grabbing. If you push the filament by hand, it will surely work.
Okay i will try that if when it happens again
If the filament does get ground down, I don't recommend pushing it by hand. That will work to solve the immediate problem, but you'll then be forcing a mangled piece of filament up into the Bowden Tube, and already tightly-toleranced teflon coupler etc. There's a good chance it will get stuck and/or do damage as it goes.
It sounds like some of you have a loose bed that doesn't come back to the same spot every time. Please experiment with pronterface:
http://koti.kapsi.fi/~kliment/printrun/
Install pronterface, connect to UM2 with USB cable, and then connect with pronterface. It should be able to pull back lots of great info from Marlin. Now experiment by moving print head up and down:
G0 Z0
That should move the bed to where the nozzle touches the glass
G0 Z0.1
That should move the bed to where you level with paper
G0 Z30
G0 Z0.1
That should move the bed up 30mm and then back to leveling area.
Try moving the head around - maybe push on the head with your fingers. Try to get the problem to repeat. I'm thinking something is loose somewhere. I think that is Nicolinux's theory also.
Maybe it's because they know much more about the machine so the can solve their problems alone.
Giving advice is so much easier than fixing it myself! When I have a problem I pretend someone else posts on the forum. I never like the advice I give myself - I think "surely there is an easier way!". 
Seriously though - I know so much because I read so many posts! Most of what I post is something that someone else learned.
Thanks for the tip gr5 i will try that as soon as i can.
Changed the filament (from colorfabb black to UM Natural) yesterday did two consecutive prints and the problem didn't happen.
Thing is that with colorfabb black i already had two prints where the filament was grinded so bad by the feeder that it was almost cut. Maybe it comes more from the filament than from the bed. I will try pronterface anyway as i want to understand more about gcode
Little update.
I had to re-level the bed because after downgrading the firmware to 14.01.02, I had to reset to factory settings. Now it behaves the same as before. I level the bed with very little resistance from the paper/nozzle. But the first layer is still squashed and ugly.
I moved the bed up and down a few times, gave it a very slight tug (hoping that whatever is loose will show itself) but nothing changes. The bed is way too close.
I will break out printrun asap and test around.
Ok, another update.
I did play around with Printrun/Pronterface and things look grim :/
There seems to be significant play with my z-stage. Check out this short video:
I used printrun to move the bed around and after homing the z-axis and entering "G0 Z0", that's where the bed was at:
No way this would touch the nozzle...
I then moved the head around (by hand), alternating between Z30, Z0.1 and Z0. On the left side I have this with Z0:
And on the right side:
And the thing that sucked most - when I tried Z0 at the center I got the optimal distance from bed/nozzle (none...)
So my z-stage shifts around for whatever reason. I checked for loose screws or other oddities but didn't find anything.
By the way, if you use Printrun, don't forget to home the z-axis first or else it won't accept any gcode instructions regarding z-movement.
I have to check my bed if it does the same.
For me i have the following. Now i'm able to identify when i'll have trouble with the bed level.
When i start the print i hear an abnormal noise.
I cancel and if i don't hear that noise the layers are ok (assuming my leveling is ok).
Still didn't have the time to check with pronterface. I resolve the issue by cancelling and starting the print again...
Two things:
1) I don't have nearly as much movement. I did the same test with tape (which will not stick to that left rod as it is oily - maybe you should add a drop of oily to the left and right rods) sticking on the back wall and a print sitting on the "shelf". I have maybe 1/4 as much movement and the z screw moves together with the shelf such that the 2 are connected perfectly. In other words all the movement (what little there is) is from the Z screw moving up and down.
2) wait so left side was about 1mm off glass, center 0mm off glass, right side 1mm off glass. Was this repeatable? If so either your glass is curved or your gantry rods are curved, right? Or is this *not* repeatable? I'm guessing it is not repeatable.
I think you should take the Z stage apart and see what is loose - probably some screw or nut needs tightening - probably something simple. Take a picture for us when you do.
Get this assembly manual for UM2 here and click "raw" to download it. Save. Then open and go to page 27 where it shows the part pretty much guaranteed is a problem for you. That part needs 4 screws - I bet there is only one left holding it together. The "trapezoidal nut":
https://github.com/Ultimaker/Ultimaker2/blob/master/um2%20assembly%20manual%20V1.1%20_english.pdf
Actually you don't have to take anything apart! On page 34 I saw this - tighten those 4 screws shown with the blue arrow. Hopefully you aren't missing any:
And by the way - the screw that the screwdriver is connected to is the one that you need to remove to slide that rear cover up. There is a similar one on the left side I think.
Good idea. Thanks for the hints. I checked the screws - none of the four are missing (and the 8 screws for the big rods are also in place).
Forgot to mention - the big rods are oily. The piece of tape barely stuck
So I guess I don't have to take the z-stage apart (not very psyched to do either...).
Checked my plate it moves a bit like yours. Not sure if more or less but it moves. Will check the screws but the four are there so that's a good start.
Danger of getting Mum to suck eggs here but in the UK all my paper is 0.15 not 0.1 so I use g1 z0.15. I have found that modifying the z-offset by <0.05 has a noticeable effect. So make sure your code and paper thickness correlate.
They should really ship a nice piece of 0.1mm metalsheet with the machine 
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I'm pretty happy with mine, the cylinder test worked good for me out of the box too. Maybe i'm one of the lucky guys who knows?
Maybe it's because they know much more about the machine so the can solve their problems alone. I would be very surprised to see one of the people you mention open a thread about an underextrusion problem or anything else as they have all the answers
Back to the topic
Yesterday i had the same strange behaviour.
Did a print everything went fine.
Printer cooled down enough so i could take the piece off with no effort.
Launched another print. Brim didn't appear or very slightly (i took a pic but nothing is seen although it's black). Feeder is skipping.
Abort the print
Touch nothing
Relaunch the same print.
Perfect brim and first layers...
What's happening?
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