Do you use default Cura profiles / settings? Can you share a few photo's of your feeder and print head? Thanks!
The Um2 "plus" (plus is key) are much less likely to get underextrusion than other printers. If it's not brand new and if this problem is recent then I'd change the nozzle and the Teflon part as tinker says above.
Also the wrong cura settings can easily cause underextrusion. You could try posting all of those - the key ones are: layer height, print speed (all layer heights and all print speeds), line width (all line widths) and temperature but that's set on the printer.
Also try a cold pull. It's complicated to explain but easy to do. A cold pull will clean the nozzle from the inside - there could be sand, dust, pieces of nylon, etc in the nozzle. Cold pull (also known as "atomic pull" or "atomic method"):
What temperature are you printing at? If too cool, that might cause some stress on the feeder
Oh and set the tension right in the middle PLA and most other filaments.
I had this with PVA filament that was damaged by moisture. If you are using PVA (or any other moisture sensitive material, like nylon), make sure to store it in a drybox of some sort (buy one, or make one yourself. Just google it )
Edited by darsan
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tinkergnome 927
Another common reason is a worn out PTFE coupler (it's a consumable). Have you checked it?
There's a picture at the end of this topic: https://ultimaker.com/en/resources/21477-how-to-fix-under-extrusion
Instructions for Replacing the PTFE coupler (from 3DVerkstan)
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