What would make a custom list type field on a customer unable to click the show in filter region checkbox when set as an available filter?

@HL You're talking about through a saved search I'm assuming? Are you using this custom field at all in the criteria of the search? Sometimes if a field is used in the criteria of the search, you cannot add it as a filter.

Yeah in a saved search. I've seen what you're talking about before and that isn't the case here, but I did try recreating the search and... it seems to work now. It's still greyed out in the existing search though. I'm flipping back and forth and I don't see any differences, so chalking it up to NS weirdness until further notice. Thanks!