Syncing misses contacts

I've been trying to sync my iPhone and iMac contacts but for some reason not all contacts are being copied to the iPhone.

I'm using the iTunes method and a manual sync. The only thing I'm syncing at the moment is a single group in the address book. Before syncing none of the contacts in that group were on the iPhone. After syncing there is still one missing. I've tried repeatedly and gone through all the settings I can find but nothing looks as if it would cause the problem. There is a difference in the information in the contact; it is the only one with a phone number but no email address but surely that shouldn't stop the syncing.

Version numbers are iTunes 10.7, iOS 6.0.1 and Mac OS X 10.6.8.

Has anyone come across this and know what might be causing it?

I still have no idea why this problem occurred but it has resolved itself.

After multiple attempts over a period of a few hours I gave up trying but did run a provider update which I was prompted for and noticed a little while after that the missing contact was there. Whether it was the update or something else I don't know.