This problem sounds all to familure with bluetooth in general.
Bluetooth technology is so hit and miss on the features that are supported, and so many phones have varying versions of bluetooth with or without disabled bluetooth features you can use with the phone.
i have had problems at times with my SYNC system not connecting to my wifes and my own phone. but mostly its because either my phone was off before entering the car, and i turned the phone on when in the car. it never likes to connect automaticly or manualy, i have to re-pair it sometimes with the sync system by removing the partner ship from the car and the phone. it's also a problem with the fact that i have my wifes phone set as the primary phone in the car. so it connects to her phone before it will connect to mine. and when her phone is not near the car and only mine is, i still have to manaly tell it to connect since it's not the primary phone.
you will find that bluetooth is more of an issue with a phone than the SYNC system. that statment is more understood when you understand how SYNC works. i have had so many problems with Bluetooth over the years its not funny. when it works, its awesome. when it doesnt, its a pain in the butt.
I have at one time locked up my edges nav unit when trying to sync with Bluetooth, and had to shut the system off completely and back on to even get it to do anything, but that’s due to the fault of Bluetooth.
Like it was said earlier, if you have problems with it, remove the relationships from both the car, and your phone. Then power both the phone and car completely off. Then back on. make sure both Bluetooth devices are back on with Bluetooth enabled before trying to pair one with the other, and remember, if it takes you too long to enter in your pair code, it will time out and you will need to try to pair it again. I have not had to re-pair both my phones on the SYNC system in the past 8 months or so but I have had problems at times with it not working as I want it to. Nothings perfect