Have you turned on AP logging from the WLC?

>config ap logging syslog level ?

alerts         Logging severity level 1.
critical       Logging severity level 2.
debugging      Logging severity level 7.
emergencies    Logging severity level 0.
errors         Logging severity level 3.
informational  Logging severity level 6.
notifications  Logging severity level 5.
warnings       Logging severity level 4.
(Cisco Controller) >config ap logging syslog level alerts ?

     Enter the name of the Cisco AP.
all            Applies the settings to all APs.
(Cisco Controller) >config ap logging syslog level alerts all?
all
(Cisco Controller) >config ap logging syslog level alerts all

All the logs will then be sent to the syslog server in the WLC settings