CSS for caption font size not working

I've successfully changed the font size on captions many times before, but for some reason, this time it isn't taking.
If I look at the source code, I definitely see my CSS in there:
.wp-block-image figcaption {
font-size: 15px;
}
But the caption size stays the same (13px). I've tried clearing my cache and viewing the page on different browsers, and the result is the same.
FYI I am using the block editor and have the GenerateBlocks plugin installed on both sites I included in the Private Information box (one where the CSS worked, one where it didn't).