Do you mean the bobbin jumps up and the upper thread winds around the bobbin, therefore jamming everything? I do a lot of decorative work, with thicker threads in the bobbin, and this was happening to me when the thread jerked. I've rather fixed this by sticking a small "jello bubble" type thingamy under the bobbin cover. You know the hot/cold clear plasticky round bits that go in the top of the taps [or do you call them faucets], well I got some mini ones from the hardware store - not sure what they were for - maybe used under items to protect timber surfaces. My newest Brother has a small extension of curved plastic beneath the bobbin cover, I'm assuming it has a purpose, but this little treasure, if put in the right spot, will hold the bobbin down enough allowing the top thread to pass over. I have to wonder if the mfs have boo-booed somewhere. Also find pre-wound bobbins do this as well when half empty.