If all of that fails, it could be a burr on a metal hook. Or if you have a plastic drop-in bobbin case, the bobbin case may be damaged.

I hope you're using good thread and not something like hand quilting thread in your machine.

And not just plastic bobbin cases ... the metal ones can get damaged too!

you have gotten good advice here. I like to use my top thread to 'floss' thru the tension discs. I was surprised to find a large glump of lint once. I hope you have already figured out your problem by now

Did you get the tension fixed on your machine? Did you try winding a new bobbin?