I'm been fighting with under extrusion on one of my UM2+ printers. Things I have tried.
1. Changed the nozzle (replaced with an Olson ruby from another working machine)
2. Taken apart the print head and cleaned out any debris
3. Replaced the PTFE Coupler (I could feed some resistance when pushing filament through)
4. Cleaned the inside of the Bowden tube
5. Taken apart the extruder and cleaned debris
6. Set the tension on the extruder to various levels between minimum and maximum tension
7. I've reset the printer to factory (a few times) and loaded Tinker V18.11.1
I am just trying to print the Ultimaker Robot from the SD card.
What I've noticed is that the printer will warm up, try to extrude some filament before starting to print (nothing comes out) then starts printing with no filament coming out. When I go to Maintenance -> Advanced -> Move Material and manually extrude by turning the wheel it takes a couple of turns but then filament starts to flow without issue.
The only thing not stock about this machine is a printed lever for the extruder to allow for manual filament changes.
I can see the marks on the filament showing that the extruder is biting into it.
Any suggestions or things to try would be very welcome.