Thanks for replying so quickly! Yes it’s in an active area, on the right side with the rest of the widgets- it’s supposed to be showing up first, on top of the others. Yes, I have other sub-categories for the parent, and nope nothing was hidden or excluded.
OK so now that I realize that the widget doesn’t show at all if there are no sub-cats for the category, I think I can figure out the problem- Selecting “Use first category of post as parent” might not be working because my posts have multiple categories? (So which one is “first”?)
I will investigate further and come back with an update just to help anyone else out there with this problem. Thanks!