In searching online I found couple of ideas that may help with this:
There is a screw on side of bobbin case that you turn. If you look on page 68 of your manual it walks you through adjusting tension. There are 2 screws you want to adjust the flat head screw NOT the Phillips head screw. Turning clockwise tightens counter clockwise loosens.
Check your bobbin case. To make sure the thread hooked into the bobbin case properly. Take the case out, put in the bobbin to make sure it's caught properly and then put it back into the machine.
Check that the bobbin tension is adjusted correctly by taking out the bobbin complete with the spool. Pull the cotton thread and it should be a smooth light pressure if the tension is correct. If it is tight to pull or pulls freely then adjust the tension screw which is found on the outside of the bobbin. The screw is very small and requires a miniature size screwdriver. The adjustment will only be a slight turn of the screw one way or the other to obtain the correct tension(depending if it was loose or tight). Once this is set reinstall the bobbin and tread up the machine, hopefully with the upper tension set at approx."2" the machine will sew correctly.
Hope these help someone