Try to factory reset this AP, delete its entry from controller and rejoin.
If still failing, try to fetch output of below command from AP CLI.
get syslog log
Check in log if you can find the reason for failure. If you are not sure what to look for, open up a support case for help.
Syamantak Omer | Community moderator | Sr.Staff TSE | CWNA | CCNA | RASZA | RICXI