I have had my 6600 going into my 7th year now. I have never had one problem with it and I use every type of thread. Yes, you do get the birds nest, but I will use leaders and enders to solve that issue. I have taught several classes and worked on a lot of gals machines during class and 9 out of 10 times it was threaded wrong. But the bunching up of thread underneath was almost always due to the bobbin being placed backwards, as some need to be clockwise and others counter clockwise. With the 6600 it MUST be in counter clock wise or shall I say, the thread has to come off into the counter clockwise motion. If not, your machine will not operate properly. It is the best machine I have, as I have two other Janomes. It is my work horse and free motion quilts beautifully. I will never part with it. Having said all that I would check the bobbin out and your threading and if you still have a problem, go back to dealer. Good luck. C.