I don't have a Juki, but I do have a Brother that has experienced the same problems. It has had the ping from every problem mentioned by all the others and from the bobbin not being the right one for my machine. The 15 and type L bobbins look similar but are very slightly different sizes in height. It would sew okay for a while then hang up, loop the upper thread, then pop. Very hard to see or find and wouldn't do it for the tech. My husband noticed that the bobbins weren't the same height while in the bobbin case. Thank goodness.