Thread out of the take up lever will do this too.

Unfortunately, I have a bit of experience with this problem.In addition to making sure your machine is threaded properly, I would recommend Little Genie Magic bobbin washers. They go in your bobbin case before you put your bobbin in. They make the thread come out of the bobbin more smoothly and cut down on tangles in the back of your quilting. They can be purchased at most online supply stores. I have also seen them at some of the bigger quilt shops.
Good Luck.

this sounds like a tension problem between the top and bottom tension, I recheck every time ichange bobbins

if you are FMQ maybe you are moving to fast with your hands , i would check the tension, and the needle

Thanks for sharing! It always helps to have a visual!

I use a hand mirror too, with a small light attached to I can check the bottom tension. Much easier that trying to crawl under the quilt, turn your head at a funny angle to see the bottom stitching.

I find when this happens my thread has "jumped" out of the uptake arm (the one with the hole in it that moves up and down as you sew). My machine also sounds different when it's stitching and this has happened. Good luck.

I have a Janome 6500 and I do a alot better with the feed dogs up. Still set it to quilt mode. I do not have it on a frame though.

I have that sometimes, but mine is much more loopy thread, with my Viking. I've figured out that the upper thread has somehow come out of the tension, when this happens. There doesn't seem to be any rhyme or reason as to when it happens either. It's very frustrating, because the stitching doesn't feel any different, until it jams from all the thread underneath. I'm not sure what the solution is, as I've had it checked when I bring my machine in for it's yearly check-up and cleaning.

Rethread top of machine. Luckily it is very easy to unsew. Other than the tension problem, it looks like you are doing a good job at your pattern. Keep up the good work,