Hi,
Is the AP on the same subnet as the controller?Are there any routers or firewalls between the AP and controller?
Is the AP is a controller based AP? If it's an Instant AP it won't automatically find the controller. You can tell if it's an Instant AP as when the AP has booted it wil start broadcasting an open Instant SSID. Also it will be reference IAP-315 on the back if it's an Instant rather than just AP-315if it's a controller based AP.
Is the AP brand new? It might be worth plugging a serial cable into the AP console port to see what's occurring. Whilst it's booting, looking at the console, you can see the IP details the AP received and if it finds the controller using ADP (or other methods).
You can also reset the AP:
Connect the serial console adaptor cable from the AP console port to your PC/Laptop
Power on the AP and get into apboot mode. You'll see the option to go into AP boot mode when the AP is booting. You will have to press Enter within a 1 second window so pay attention to console messages during bootup.
From the apboot prompt, enter the following commands
apboot> factory_reset
apboot> purgeenv
apboot> save
apboot> boot