Inspired by this post, but also because I've seen other users get stuck because of the same issue...
When the Resource Manager is set to filter a specific type of Resource, but none exist in the file that's being viewed, there is a message which simply reads "There are no items to show in this view." Because this isn't particularly helpful, there should also a button which says "Switch to viewing all resource types" that you can click to switch the filter to All Resources (same as the dropdown). Or something similar.
I'd use it too, even though I'm familiar with the dropdown - it's faster and more apparent.
Note this would also be applicable to searching the RM, when there are no results in the filtered resource type, but there are results of another kind.