Capture is used to sniff traffic flowing through the ace itself.
What you were probably looking for is the command "debug".
try -> debug aaa all
This will show you all AAA related traffic/events.
Be sure to remove the debug condition before logging out with "undebug all"
If you want to capture traffic you need an access-list.
Example:
conf t
access-list All permit ip any any
exit
capture all access-list All bufsize 2048 circular-buffer
That should help you capture/sniff traffic passing through the ACE itself.
Capture and Debug are two different things. :)
Hope it helps
Roble