check that the thread is going through the tension spring correctly, you may need to turn the screw in the bobbin casing to tighten a little, or loosen the top tension. It would depend on how much has been sewn whether you try stitching over the top. I imagine the design would end up rather stiff if you do the whole design. You could also use the same colour thread in the bottom as on top, and it may not be as noticeable.