I am a long arm Gammill learner and I have encountered this problem and we found that the bobbin was not in the casing properly. I had to unquilt a couple of times before we decided to to come up with the following:
Now we are making a sample of quilting on the side of the quilt, meaning on the excess of material that we use to clamp, using a piece of material similar to the quilt top, and it is easy to check with a mirror or by folding the edge up to check it up. We do this every time that we change the bobbin and before we start quilting on the real thing.
You could also go to the back of the machine and feel the stitches with your hands underneath before you proceed with more quilting.
It seems to work for us. Your quilt is very nice, the quality of quilting should go along with the quality of the top. Good luck and have a good party.