Have you added new users or rooms in Exchange and have users say they cannot find them? The easiest trick for me is to have the user rename their OAB folder.
1. Close out of all Office products including Lync
2. Click on Start and paste this in: %userprofile%\Local Settings\Application Data\Microsoft\Outlook\Offline Address Books
3. Now rename the file there with a .old at the end ..example 81bc7ac3-6618-4c53-b711-f647e4f4a.old
4. Open Outlook and give it a few minutes while it re-downloads the address book