The "thumb" I was talking about is not the one associated with the bobbin case; it's a piece of the bobbin case housing within the machine.
Here's a fw troubleshooting article I found on a quick Google. It shows the correct positioning for that metal "finger" I tried to describe:
http://www.suite101.com/content/comm...problem-a50918
However, an even more common problem is threading the needle in the wrong direction.