Please read this document:
http://www.arubanetworks.com/techdocs/ArubaOS_61/ArubaOS_61_UG/VRRP.php
The preemption option will allow the APs to come back to the main master but you have to be careful when enabling this because if your main controller is having issues coming up and down then you could create ping - pong effect with the APs going back and forth between controllers , add the following :
tracking master-up-time 30 add 20