It seems to be an issue with VAG group headunits/BT. A lot of people are having problems getting the phonebook to sync (I am having the same problem on my One and the Sensation before it). I've been told that it's something to do with more recent versions of Android not supporting rSAP although whether this is true or not I don't know.
I've found a way around that works for me though (the phone interface of my headunit is not the best either) and have bough a Brodit holder - when I put the phone in it I activate the "Car Mode" on the handset and I can access my phonebook through that a lot better than the headunit would allow.