Oh and one last thing. Sometimes depending on the material, one of the factors above was temporary such as temp and speed but it caused grinding. This causes the knurled bolt to be packed with ground material which made additional grinding/slippage more likely at only slightly too cold or slightly too fast printing.
I am printing with Ultimaker brand filament, 30 mm/s, 220 degrees Fahrenheit nozzle temperature, 54-57 degrees Fahrenheit room temperature. So I think that rules out a lot, what is left is partially clogged nozzle, deformed nozzle elements, and feeder pressure is too high.
How narrow down which of the above 3 is the cause? I think maybe I pull the feeder latch too tight, but I am not sure what is too tight and what is just right?
That nozzle temp better be 220C, not F. :???:
Have you tried running filament through with the bowden disconnected from the hot end? If not, do this to reduce some of the variables. Put some resistance on the filament with your fingers. See how much it takes to create grinding. Then try pushing some filament manually through the hot end and see what resistance is there. See if you can get a good line that way, or if there is evidence of a clog.
Yes, I meant Celsius for the Printer, but Fahrenheit for room temperature, American habit. 
I am able to manually push filament through the hot end, doesn't seem like too much resistance. Even with the bowden tube on, I can still move the filament up and down using only my pinky finger and thumb grasping the filament, albeit with some mild resistance.
some filament is duff - check other filament types.
For tightness I open it up all the way, and then tighen it until I can't pull it out with light finger pressure - i.e. not very tight at all.
If temp is good, nozzle clear then it does not take much push to get everything to work.
I have some filament that was good but now prints for about 3 hours before it cloggs, reduces flow and goes fluffy. I know it is the filament because I have cleaned, put it back in repeat - same thing, whereas when I clean, put in different filament - all good for 12 hours +
It seems on year old filament this can happen - so now I prototype in old filament and keep my stocks lower. End up with some fun prototypes 
Sounds more and more like a partially clogged nozzle.
Get the printer up to temperature and unlatch the pressure arm and see how much pressure it takes to extrude...
Also check out the atomic cleaning method which is basically allowing the filament to cool down to near glass temperature and then pulling it back out.
https://www.google.com/webhp?#q=site:umforum.ultimaker.com+atomic+cleaning+method
Also note that proper leveling is critical to prevent grinding on the first layer. Too little distance from the nozzle to the bed will prevent extruding and build up pressure until the feeder can't feed any more filament.
So the atomic cleaning method for PLA is basically insert filament at 210C, cool nozzle to 90C, the pull out the filament hopefully with any plastic in the nozzle? Is there is of damaging the nozzle?
If you are as strong as me then pulling as hard as you can won't damage the bowden or anything. But a good rule of thumb would be 10 to 20 pounds should be enough. If it comes out with less than 5 pounds pulling force nozzle was too hot. If it takes more than 20 pounds force, heat it up 5C to 95C. Wait 10 seconds and try again. If you still can't do it after 30 seconds at 100C then something is wrong with your heater. Or you have strange filament that needs to be printed hotter than normal PLA.
too hard to pull can also mean deformed teflon insulator in the hot end. the part inside the PEEK touching the brass extender can get deformed if you've printed things like ABS or XT. filament starts to get in and trying to pull out the filament during clean takes a lot of force. it might also be the cause of the op's problem.
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anon4321 16
I've found the UMO feeder to be fairly reliable. If you are getting chewing, it could be due to a few reasons:
Printing too cold
Printing too fast
Printing too cold and fast.
Filament might be too large. Measure with calipers, needs to be 2.85 or thereabouts. If it is more than 3mm, it can jam. Measure it at different spots as well as at different angles around the filament in case the filament is oval.
Nozzle could be partially clogged.
The plastic insulating elements in the nozzle are deformed from too much heat.
The feeder pressure is too high squishing the filament into more of an oval and the oval shape at it's widest part is wider than 3mm.
If the feeder pressure is too low.
In my experience, it has been too cold or too fast or both and occasionally it has been a clogged nozzle.
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