Are you trying to start it after a restart? If yes, try powering down completely first. Be sure you're holding the D key down (until you see AHT) and not spamming it. Be sure you're pushing the key quick enough. Must be before the grey screen pops up. Be sure you have all of your other peripherals unplugged (external HDD, RAID, external ODD) only have the keyboard, mouse, display, and ethernet, nothing else. Hope this helps!
Integration & Warehouse Manager
Maxx Digital http://www.maxxdigital.com