Some things to check -
Be sure the top thread is correctly threaded, especially around the tension disk.
Be sure to clean thoroughly under the needle plate.
Be sure the bobbin case is clean.
Be sure the bobbin itself is wound correctly - it shouldn't be "mushy" and it shouldn't be too tight.
Try a different bobbin, a different thread (top and bottom) and see if the problem persists.
What kind of longarm are you using? If you can call a dealer or the factory for help, don't hesitate to do so.