My neighbor just called me to come check her sewing machine. She has a Bernina 801. Great little machine! Her complaint was that it wasn't feeding the fabric through. When I got there, here's a list of what I found.
1. The feed dog drop switch was set for "darning," but the feed dog height was also low. So we moved the switch to "sew," and raised her feed dog height.
2. The needle was in backwards. We put the needle back in the right way, with the flat side to the back.
3. The bobbin tension was way too tight, and the upper tension was set loose. So we put the upper tension on its "normal" setting, and loosened the bobbin tension. Then I did a sew off sample, and balanced the tensions by adjusting the bobbin tension.
4. When I asked for a different color thread for the top thread, she handed me some "hand quilting" thread. I told her to NEVER use "hand quilting" thread on a sewing machine. You CANNOT get a good stitch with "hand quilting" thread. And you may get a burr when it tangles and pulls the needle into the hook.