[JamesC70]

If the uploaded signature is too large, then the animation may be lost when it's resized to fit whatever space you've allowed for animated siggys.

Verify that the user is allowed to upload animated signatures. AdminCP -> Usergroups -> Usergroup Manager -> (select applicable usergroup) -> Picture Uploading Permissions -> Can Upload Animated GIF for Signature.

Also verify that the allowed height, width, and filesize are reasonable for the signatures allowed. (My forum uses 300w x 225h x 16383 bytes (16k).)

The user(s) in question will need to re-upload their siggys, as changing these settings will not magically revert non-animation back to animated GIFs.