The bobbin thread showing is a clue. If there is not sufficient tension on the bobbin thread the stitch can be loose on top. You didn't say which bobbin case you are using, but have you tried the other one? Some bobbin threads can shed fibers that builds up behind the tension spring in the bobbin case, pushing it out so there is less tension. You can compensate for it by reducing the top tension, but means the overall stitches are looser than they should be.