I wanted to post this here since I came across this thread when searching for the problem too. This is a known bug on the IOS platform with RTSP and NAT ALG. Here is the link to the bug:
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCtn86205
The ...
The command that you are looking for is if "aaa server radius dynamic-author" is configured. The RADIUS implementation on the IOS device won't be listening for COA messages otherwise.
