Get a piece of thread and hold it like dental floss (actually dental floss might work!) and pull it back and forth between all accessible parts of the tension discs to see if you dislodge a piece of thread or fluff.
Tension discs work like two saucers base to base with an adjustable coil spring holding them together, but if you use a twin needle one of the threads has to go either in front of the pair of discs or behind them. That's why I suggest flossing all places.