Re-thread your LA using the manual to guide you. I spent an entire day trying to fix a tension issue, looked at the threading of the machine at least 100 times that day and did not see that the machine was threaded incorrectly. Next day I used the manual to check and my threading mistake jumped right out. After I fixed it, my LA forgave me and sewed wonderful stitches all day long.
Check your bobbin case for lint and caught bits of thread. It's amazing how much havoc can be wreaked by a tiny bit of lint/thread caught in the thread guide of the bobbin case.
Make sure you have the bobbin in correctly. Again, check how to insert the bobbin in your manual. If you did it wrong by mistake and you're like me, you'll never notice the mistake because you're 100% sure you did it right, so that's what you see - even when you did it wrong.
Take your needle plate off your machine and clean thoroughly, then oil the bobbin race as described in your manual. Clean excess oil off so that you won't stain your quilt sandwich.
It cold be a timing issue. Timing is the interaction of the needle with the bobbin mechanism to bring the two threads together to make a stitch. I don't know how to check that other than to get your machine serviced.
Good luck to you. I know how frustrating this can be.