Gravatar has been playing up a great deal lately, resulting in many threads about the problems. e.g:
http://www.theanswerb.../Question1074183.html
So there might be nothing that you can do about it. However people often had problems getting avatars even before the latest round of problems, so you could start by checking that you've not made any of the 'usual' errors:
1. Check that you've used the SAME email address to register with AB as when registering with Gravatar; it's ESSENTIAL! ;
(If you're unsure of which address you used to register on AB, go to My Profile > Edit My Profile).
2. Check that you've rated your avatar as 'G' on the Gravatar website ;
3. Check that you've actually activated your avatar (by ticking the box under Edit My Profile).
Chris