I think Candi is probably on to something. The invisible thread is so fine that if you try to sew with a needle that's too large or has too big an eye, the thread doesn't form a loop large enough to be caught by the hook. Skipped stitches.
Try a size 70/10 or even a 65/9 and see if that doesn't clear up your problem.
Since you can't adjust the upper tension, you may be able to compensate by tightening up slightly on the bobbin tension. Good luck! :)