The top thread is the problem. I used to have this problem frequently when doing FMQ and still have it at times. I can't exactly say what causes it because I'm a novice at FMQ. For me it tends to happen when I get my hand speed and machine speed out of sync. After much frustration and trying every possible solution, I have found when it happens now I need to walk away. After a bit of a break, things always seem to go better.
I love the machine and hope you will come to love yours, too. I'm no expert on anything, but I've had the machine for over year. Feel free to PM me if you have any problems that I may be able to help with.