I have several AP that can't associate with the controller.
When I use the debug lwapp option and when I'm connected in console on the APs, I can see that the AP don't send the LWAPP discovery request.
If you are able to console into the AP and do not see the discovery process, check to see if the AP's are autonomous and not LWAPP. Enter any lwapp commands like : show lwapp ip config. If you get an error then you got the wrong ap or you will need to convert to lwapp. Better yet, do a show ver and see.