Yes to everything above, but also this:

I've done this in the past when it was a case of operator error. Pull the manual out and read the instructions. Pull all the thread out, and rethread the whole thing, making sure you hook the thread through every single little hook. Missing one will stuff it up. Also, rethread the bobbin and check that it's in the right way. Finally, check the tension, make sure the feed dogs are up/down (appropriate to what you're doing), and make sure the presser foot is at the correct height.

If you've tried all of that, the problem may be that the timing is out on the machine, in which case it might need to go to the shop for a service. It will purr like a pussycat after a bowl of cream when it's had a service.