Hi,
1) If the debug client doesn't show anything it is either because the client is not associated on that controller or that nothing special is happening (client is just working fine, not roaming, nothing).
2) Every 30 minutes sounds like a session timeout. By default the WLAN session timeout is 30 minutes, did you check that in your WLAN setting on the controller ?
Hope this helps.
Nicolas
===
Don't forget to rate answers that you find useful