The client uses its SSID to identify and associate to the wireless network, so ensure that the SSID is configured identically on the WLC and the client. In order to check the SSID configured on the WLC, click the WLANs page. Click the appropriate WLAN, and check the SSID configured under the General tab.
SSID is case sensitive. It might help the wireless client to associate to the WLAN if you delete and recreate the WLAN.
A Service Set Identifier (SSID) mismatch is the number of times a packet is received by the radio that contains an SSID field that is not matching its own. This applies to probe response and beacon packets, and indicates the possibility of some coverage overlap between two Radio Frequency (RF) cells having different SSIDs.