When you use Windows Update, Microsoft Update, or Windows Server Update Services (WSUS) to install updates, you may experience one or more of the following issues:

  1. You are offered an update for a product, but the update is not downloaded or installed on your computer.
  2. You are repeatedly offered an update, even if you have already installed this update multiple times.
  3. A particular update failed to install.

KB906602 discusses how to troubleshoot these issues.

Also check out this post: Windows Update Troubleshooter from Microsoft.