My contacts will not sync with my Macbook Pro

My contacts won't sync between my iphone 5 and summer 2012 macbook pro. I have enabled syncing contacts when my iphone is plugged in and itunes is open. It seems to transfer contacts from the macbook to the iphone but not the other way round. So if i add a contact on my phone when i next sync my phone it will not transfer the update to my macbook.