Assuming that you can see your avatar on the Gravatar site, click on 'My Profile' here and then on 'Edit My Profile'. Take a look at the email address you used to register with AB. The address you used to register with Gravatar MUST be that one. (It's the only way that your avatar is linked to your AB account).
Also check that there's a tick in the box which activates your avatar.
PeaBrain, you're not the only one to get stuck here, I've wanted to change my avatar for months now and it just won't happen for me.
I'm leaving it alone for a little while or I'll end up in the loony bin.
Every one else seems to change 'em every 5 minutes. Grrrrrrr!!