Take out the bobbin case, if the machine has one, and check for any caught threads. Reroute the thread carefully and try stitching while holding the threads for two stitches. I find more bobbin problems that top problems with the problem you describe. Also rethread the top thread paying attention to alignment between the tension disks.
If you promise not to laugh, I'll tell you what I've done more than once - skipped the thread take up hole when threading the top. The result is what you describe, but I'm sure I'm the only one who does that. In my defense, the machine I do that on has the thread take up around toward the back.