It's probably something very simple that you've overlooked. For me, it's usually the threading order that I have to repeat from scratch when a thread has broken, and as long as I've had my workhorse, I still have to look at the book. Have you re-seated the needles, have them up as high as needed and turned the right way? I wasted a whole get-together and even more frustration at home until I realized that the knife I'd replaced had not seated properly, even when I did it over and over.