Assuming you do not have a CCM issue, try disabling all CAC and WMM. I think there might be a bug with the 7921 and WMM/CAC. We have been having problems with it and TAC hasn't gotten them resolved yet.
No issues w/ CAC/TSPEC and 7921. If using autonomous AP, ensure 12 mbps is set as a nominal rate as that is the PHY rate configured on the 7921 by default. If using WLAN controller, ensure 12 mbps or higher is enabled.