It’s a bit hard to tell what’s going on without being able to log in and see exactly what your users are seeing.
I suggest logging in as a non-admin, then right-clicking the space where the avatar should appear. Select ‘Inspect Element’, and then browse through the developer tools to see whether the avatar is actually in the source. If it is, but is hidden by CSS, you should be able to use the inspector to figure out which CSS is the culprit. If it’s not there at all, you could have some other sort of problem – like, perhaps, a failure to connect to Gravatar.