So, it turns out that I don’t need a new bobbin case after all. I wanted to check the timing one more time, and came across the Sew It Works! Timing test page.
Apparently, when I changed the timing hook gears a couple of months back, I didn’t quite get it right. Then, when I went over several layers of fabric, it jostled things enough to throw the timing off. The timing test outlined on this site is fantastic–it’s the clearest explanation of where the hook needs to be in relation to the needle that I’ve seen. So, I tweaked the hook gear, got the timing right, and finished the bonnet.
Now, something else is loose because just as I was finishing the bonnet, it went way out of timing again, as in, the needle hitting metal. I’ll have to open up the machine again and figure out what I forgot to tighten that’s throwing things off. At least now I know how to get the timing right. And all sorts of other things, like change the bobbin tension. All part of the learning experience.