Have a unleashed group of 5 AP working for one year today added 2, first one no problem the second one doesnt join the group I can ping the AP from the admin web page, approved the new AP but keeps saying it is disconnected, If I go to http://ip of the new AP, the stand along page loads, all the APs including the new one have the same Firmware version.
If the stubborn AP is on the network switch like a working one, I suggest doing a Hard Reset on it. Does the AP pull a DHCP address, such that your laptop on that network can ping it? And then, check your Master AP access point page and does the new AP ever appear listed there? Let things go for a little while and does it ever appear there?
Upgrading the current APs to this latest release, might help your stubborn one to be recognized, or push the same onto it manually, and see if it connects with the others then? Good luck and best regards.