Are you using the same port as the other APs? Are you sure it's getting an IP address?
If you are sure it's getting an address and it's on the same VLAN as the other APs, purge it and see if it comes back up. To do that, hit the Enter key when prompted during the boot, then type "purge". After that finishes, type "save". Now reboot the AP and see if it connects.
If it doesn't, like Colin said, post the boot log and let us see where it is breaking down.