Originally Posted by Barb in Louisiana

Sew a seam on some fabric and see if your top seam pulls out easier, or is it the bottom, bobbin thread. This will help find out what part of your setup is the problem. Ironically, if the top thread pulls out too easy, the bobbin needs to be looked at. If the bobbin thread pulls out too easy, then it could be your top thread not going through the tension discs properly.
One thing I forgot about. If you are using a long stitch, say 2.5, it could cause a loose seam. I usually sew with 2.0 or 1.8. The seams are harder to rip, but stay together better.