your tension is off, try turning the tension knob just a little bit, like 1/4th turn at a time and sew a little bit to check until the stitches look the same on the top and on the bottom...the bobbin thread should not be able to be pulled out at the end of a seam. there should be no sign on top thread on the bottom or bottom thread on the top when your tension is set correctly. good luck, it sucks to have to deal with tension issues!