Not sure if this will help but I'll share my experience with the same problem. In my case I changed FROM ABS TO PLA, and some of the ABS remained just below the teflon above the nozzle. Took me a few hours to figure it out. I basically removed the teflon piece and tried to pass a piece of PLA through it and found it was too small at the bottom. Since the ABS was white, I had no chance to see it stuck to the teflon.
Because of the partial obstruction, there was too much back pressure at the extruder. Once cleaned out, it worked like new again.