Are you using prewound bobbins or are you winding your own? When I have issues like that I wind a new bobbin using a different bobbin, I change my needle, rethread my machine, and give it a good cleaning and a drop of oil, and start sewing on a scrap till I’m confident my issue has been fixed. What machine do you have? I also use the same thread top and bobbin.
I’m sorry your having issues, someone with far more experience is sure to stop by and offer better advice.