A few things to check. (1) Do you have the quilt too tight in the frame? The rule-of-thumb for frame quilting is that you should be able to grab a finger pushing up from the bottom. You need a little sag in the quilt to provide "give" as you sew. (2) Run your finger around the needle hole to see if you can feel a burr. Also run your finger around the inside of the bobbin race, and around the outside of the bobbin case. If you find a burr, you can sand it off with an extremely fine sandpaper (such as emery cloth). (3) Are you sure you have the bobbin unwinding in the correct direction? You can try switching the direction, but go slowly and carefully so you don't create a jam if you are wrong. (4) Your top threading may be wrong. You can experiment with this.
(5) You say that tension has been figured out, but I wonder if that is indeed the case. I would *strongly* recommend that you watch the following video:
https://www.youtube.com/watch?v=Q1mRhcquZTM
You may indeed have the tension correct for a balanced stitch, but the entire tensioning system may be set too high. You want to set the bobbin tension as loose as possible, then set the top tension to create a balanced stitch. If you have not set the tension in this way, both tensions may be too tight.
(6) What size needle are you currently using, and what type?
(7) And what brand of thread? Some threads are better suited to longarm quilting than others. I would highly recommend trying Glide thread. I have had other threads break or shred in my midarm setup; Glide never does.