Could be a burr on the hook or needle plate. Or could be the timing is a bit off. If you could take off the needle plate, turn the handwheel to see where the needle is going. Then you could see how close the needle is to the hook as it passes -- maybe the hook is too close to the needle, and cutting the thread or breaking the needles. But those needle plates aren't very consumer friendly. Might need to take it to the shop.