It's also possible that you have too much space between your rows. It's sometimes hard to learn how to accurately advance a row of stitching, as in a pantograph. Don't be tempted to try to correct this by tightening your quilt sandwich drum tight. That's not the problem, and over tightening the quilt sandwich will only cause bad stitches which look like thread tension issues (but are really a too tight quilt). The quilt sandwich should have all the layers smooth and flat, but the entire sandwich needs to be a bit loose in the frame, so the fabric can flex while the needle moves in all directions through it. Sewing machines are designed to stay still and sew in a straight line, so when we longarm, and have it moving and sewing in all directions, something has to give. Now the needle can't give (flex), so the fabric must. Or you have horrible stitches. I always push a finger up from underneath the quilt sandwich, and make sure my other hand can grab the end of that finger, quilt and all. Then I know it's loose enough.