I can't help you since I hand quilt.
Welcome to the Quilting Board!

Happens to me when the bobbin is not in correctly. Turning from the wrong way perhaps?
My Janome dealer 's answer to a similar question of mine re knots on the back of work was..." Always rethread the top thread when troubles occur. It may have inadvertently jumped out of the tension disks ." It seems to work for me. Pressure foot down is always another issue although my machine won't sew if this is the case. I hope this helps.
I also use gene washers...it works great...makes a huge difference preventing that issue. Another thing I do is cover the feeddogs, keep them up...I use the slider but you can use a business card or some people here use oven liners. I bought one yesterday but have not used it yet.
It's happened to me too at the beginning of FMQ, and in my case it was because I didn't have the quilting presser foot down even though I was holding the two threads. Hope that's all it is in your case.
A fabric stash is always missing that one fabric needed to finish the quilt on which you're working.
It was the presser foot. I just tried it again, and while my lines are still horrible, my threads stayed where they were supposed to. Thank you!!
When ever I forget to put the presser foot down, my machine beeps at me!!! and will not move
Amythyst
I am just starting to learn the FMQ. It will definitely be A LOT of practice....I was playing around with a scrap piece again yesterday and after about 30 minutes of frustration....I discovered that you have to put the presser foot DOWN. It made a BIG difference.....but I still need a lot of practice to master this skill. I have watched a ton of videos and I can't remember hearing that little bit of very important info??