contacts won't synchronize

Actually this is a mixed question since iMac and iPad are involved.

Contacts from my iPad and those from my iMac won't synchronize. I use iTunes and have checked the option for synchronizing contacts. When I synchronize, the status box tells me that it synchronizes contacts, but it doesn't. Contacts from the iPad don't show up in the contacts on the iMac and vice versa. Synchronizing photos and music works fine.

I synchronize using the USB cable. It's a first generation iPad and a 2010 iMac. I have OS 10.6.8 on my iMac and iOS 5.1.1 on my iPad.

What could be the problem?