Not only check tension but be sure you have a good sharp needle(schmetz- quilting needles really do a great job), bobbin is not overloaded, you are using a good quality thread- Coats and clark, sulky etc. Keep even pressure and tension with your hands and watch the fabric is not bunching up. If you are using a synthic or high loft batting that too can cause issues. Depending on the fabric- like sheets which tend to be a bit more touchy when sewing can cause thread jams. ry a few things then stop by a local quilt shop fr advice- when they see it they may be able to give better advice.