This can happen because of a few reasons, but I'm suspecting this is your case:
The original GIF size is more than 100x100, in this case vB will auto resize it to 100x100, and rendering the GIF to "freeze", because it's not properly saving the frames sequence. You need to resize it manually and properly. See below if you don't know how. Be wary of MH's avatar limit of 100x100px in dimension and 50kB in bytes.
-- If you have Adobe ImageReady, just Image > Image Size, and input the new size, save as GIF
-- Macromedia Fireworks, ACDSee, GIMP, Jasc Animationshop and Paint Shop Pro Animation Shop are able to do just the same. GIMP is obviously the best choice since it's free.