Don't know your other suggestions, so don't know if this is a repeat.
1) make sure you're threading the bobbin in correctly
2) make sure you have your needle in correctly and that it's pushed ALL the way up (you DO have the right needle, right?)
My Pfaff won't pull the bobbin thread up when the needle is in wrong. (and it's SO easy to put it in wrong with that machine!)
Leave your bobbin area open for you to watch, hand move the wheel to watch your needle go down --- does the bobbin thread move up and around and look like it's trying to catch? If not, then the spring in the bobbin case may be broken.