I have a 5 year old basic model Kenmore. While I'm FMQ, the bobbin thread will slip out of the delivery eye of the bobbin case.
Everything, from what I can tell, is right. Threaded properly, tension right, feed dog down, etc. It doesn't always do this and I can't recreate it. I'm in the middle of FMQ a runner right now and I got 1/3 of the way through with no problem. The thread slipped once and now it keeps doing this every 12 inches or so.
I just finished a lap quilt that I had this exact same problem. It is driving me crazy.