Had problems with tension also. Finally tried changing my bobin tension. Had the machine for 35 years and didn't realize that there was a bobin tension. Amazing the difference.

try cleaning the machine really well, especially in the bobbing case area and a new needle. sometimes the thread does not get (pulled) into the tension disks, this also can cause your problem the thread just laying on top of the disks and not between them. give it a good tug when threading to make sure the thread is securily between the disks.