Assets not showing up in media tab

Trying to add a particular item (#1) as a thumbnail to another item record (#2). However, even though both items are in the same item set, #1 doesn’t display in the list of assets accessible to #2.

Any suggestions?

Thanks so much

Issue resolved: problem turned out to be browser-related. Specifically, Chrome. Although as the problem was only with one person’s browser, it is more likely a caching issue.