Like Calendar conflicts, Contact conflicts in Outlook can cause sync problems. In Outlook, view ALL your contacts in a List View and look for a Yellow icon on the left instead of the standard contact icons. This indicates a conflict. Simply open the contact and choose one (typically the one with the later date) to keep.

While Contact conflicts are rare, they do happen.

The other option is to scan your inbox. Depending on your setup you'd need to run ScanOST.exe or ScanPST.exe to fix any problems that your OST or PST may have.