I do not understand why but I have learned a lesson. I bought a cone of thread (polester) and had trouble with the tension. Got another machine same thing happened. Changed to 100% cotton thread and everything started to work right. This is the short story, I have read alot of info here about thread and I have tried alot of things but bottom line is my machine does not like poly. It is a new Smarter by Pfaff it is smarter than I. Maybe I should say it does not like cheap thread. I still don't understand a machine is a machine and should not know anything.