It looks like I wasn't moving the pen enough. As I would keep writing, the shimmer would collect in the feed until it just straight-up clogged. I didn't realize I had to periodically roll the pen around even while I was actively using it. This should have been obvious by the fact that my writing kept getting more and more, well... shimmery before the ink stopped flowing.

I'll try this approach in the future.

Rookie mistake, but in my defense, I didn't even know that shimmer inks were even a thing until late last year.