Having done a lot of prints very recently, many being fairly long in duration (up to 16 hours), I have noticed that there are quite a few things that can go wrong around the print head itself!
On more than one occasion I have had a failed print due to under extrusion and there are two main problems that I have observed:
1: when you change the material, it heats up the filament to allow it to retract it all the way back down the bowden tube. Sometimes (not every time) it leaves a very fine string all the way back down to the hot end, which can snap at any point on it's length. I have found in the past that some of this fine string has been stuck in the bowden tube, adding just a little extra friction, which is really not helpful! You should make sure you blow out the tube with an air can from the hot end (detach obviously first from the hot end).
2: my UM2 gets dirt build up inside the hot end which needs cleaning out every 1/4 spool or so! I follow a set of instructions from "iMakr.com" which is my local supplier. The basics of the instructions is the you first remove the filament, then the bowden tube form the hot end. Then you heat up the hot end to 220 deg C, and when it is at temperature, insert a clean (not imprinted from the feeder wheel) piece of filament till it comes out of the nozzle. Then, with the filament still inserted, reduce the temperature of the head to 90 deg C. When temperature is reached, pull out the filament. If your hot end is dirty, it will come out with black or brown bits stuck to the filament. Repeat the process till the filament comes out without dirt.
By checking these two things regularly, I keep my printer going almost without failure. There is the odd time when I haven't checked the nozzle in time and it's got dirty again, but cleaning it usually fixes it.
I have also noticed a difference between suppliers! FormFutura PLA performs differently from Polymakr PolyPlus PLA, and requires different settings!
I hope these suggestions help.