it could be anything or a combination of things. I suggest you change only one thing at a time to find out.
First ... put a scrap quilt sandwich together - don't do your tests or practice on the quilt. 9x9" is a good size to practice on.
Then change one thing at a time.
Start with the thread. Get a good or decent quality machine thread (NOT a "hand quilting" thread). Make sure the machine is threaded properly. Try to get something that is a 50wt - which is a pretty basic weight thread. Make a new bobbin with this thread and make sure the bobbin is loaded properly.
If you still have a problem, change the needle. Make sure you have the right size needle for the thread you are using. A good needle size for a 50wt thread is 70/10 - again it's a pretty basic size needle.
If you still have a problem ... then come back here and tell us where the thread is breaking. Is it breaking before the needle but after the tension discs? Is it breaking before the tension discs? Also, let us know what kind of machine you have. And, where are the knots? Is the upper thread knotting or the bobbin thread??
I would start with the thread, and then the needle. But try ONE thing first and rule it out before you move to the next 'fix'.