Try threading bottom thread through little hole on bobbin case if it has one. My bernina works better with some threads when I do that,
completely re thread top and bottom from scratch. Sometimes this works with my singer, no idea why .
Do you have same thread top and bottom? Neutralise everything and go slow increasing and decreasing top then bottom until you find a good stitch. This I would only do as an emergency as it takes ages moving one at a time and keeping note.