Drop item disappearing after Continue (Storyline 360)

I'm hoping someone can find the needle in the haystack that's been eluding me.

I've got a quiz question (see attached sample). No matter what drop item is in the top hit spot, it disappears when the Continue button is clicked. Everything else works as should.

I've tried cutting and repasting objects and triggers in case there was something "stuck." It's only an issue with whichever drop item is at the top.

Can someone help me figure out what the issue is? Thanks!