Hi, you need to make sure that the primary WLC that you have configured on the AP is in fact replying to the discovery request
of the AP.
If the AP receives a discovery request from the WLC that is configured as a Primary then it will join it.
Some of the reasons for the AP no to join the primary are:
1- Wrong WLC name on the AP configuration
2- WLC doesn't reply to the discovery request of the AP
3- WLC is already full
4- AP is not getting authorized on the WLC and Authorization have been enabled for the APs on the WLC.
5- ....
Hope this helps
Serge