I'm coming just with the same, it is driving me weird... I thought it was a dirty nozzel, I've clean it and still happening. I'v been dismounting the feeder because ninjaflex got really stuck and from there problems. You will think, this idi*t mounted wrong, nope, becuase after that I got another perfect part with PLA yesterday.
Steps:
PLA: (almost a full spool) no problem.
(try once with ABS but not working and not trying anymore)
change to:
Ninjaflex semiflex: No problem, perfect finish quality.
Change to:
PLA: Perfect quality
Change to:
Ninjaflex (normal): gets a knot in the feeder, dismount, try again, it take it but never print, just random bubbles in the nozzle.
Change to:
PLA: a bit of trubble but finally 2 hours printing.
TODAY: not printing anything with any material.
Clean the nozzle: heat until 260 prush and pull with PLA, get some black abs bits and a weird form Photo1. Put some light in the hole look from the top and I can see straight through...
Try to print again. Same problem. Push material by hand and go out like on hits and when I pull back I get this form on the filament.
Photo1:
Photo2: