If you have done stippling before and had no problem then it could be that your needle isn't always picking up the bobbin thread. That could be for a few different reasons. Needle not in correctly, bobbin or top thread not threaded quite right, you've bumped your tension control knob, the thickness of your quilt is too great. But if it is only happening on curves then it is you that has gotten out of sync. That happens to me every now and then. When it does I get out some scrap fabric and batting and work through it until i get myself under control again.
