On my '11 I found it seems to only store 1 phone. So you have to delete the "old" one first. A device name change I think also causes pairing loss.
On my phone (GS3), I have to go to the BT Settings for the device and click it to get it to re-connect. Some ROMs seem to work better than others with this radio. My old DINC would auto-connect 100% all the time. I think its a crappy BT implementation and pray for a f/w update someday.