I thought things were going well. I was stitching faster, so it should have been turning out better than that. Thankfully, I checked the back well before I had 2/3 of the block done. Out came the seam ripper again. Unfortunately, since I was stitching faster, those stitches were a lot smaller than Monday night's stitches, so it took longer to rip out.
While I ripped out stitches, I remembered having this problem before, with one of my earliest feathers, when the bobbin was close to empty. So, I filled a new bobbin and tried again. It worked!
I finished 3 blocks and a whole row of sashing feathers, so now I'm working on vertical row 8 of 15. The funny thing is, I emptied the bobbin later in the evening, with no hint of a problem. Go figure. At least now I know to check the bobbin next time I'm having problems.