I'm not sure I agree with the back not being pulled tight enough before you basted. Sounds like tension trouble to me. If your bobbin thread is being 'unsewn' easily, that means it was not pulled up into your sandwich enough, thus the top tension needs to be tightened. This will pull up the bobbin thread more. I do agree that you make a practice sandwich using same types of materials as your quilt and practice on this until you get your tension right.
Good Luck!