Well, if an AP could not connect to the controller I would check these first:
-CPsec is enabled and the AP's are not in the whitelist - you can check this on the controller
-There is no valid license on the controller - you can check this on the controller
-The AP can not find the controller: this can be a network problem, for example they do not get IP address or some cable is not connected, acl's blocking the required ports or protocols, wrong network configuration on the controller etc. (if there is only L3 connectivity amongst the AP and controller you need to use DNS or DHCP option to tell the controller IP to the AP or use static configuration on the AP) - you can check whether the AP's get IP address on the DHCP server, you can try to ping that from the controller at first
-There might be static configuration on the AP which containts wrong information (for example the master IP is wrong) - you can check this if you connect to the console port of the AP and use the printenv command in the AP boot menu (purgeenv - saveenv resets it)
-Try to examine the console output while the AP is booting. You will see if it gets correct IP address and finds the controller