After SPS installation, most likely you will begin customizing the site groups – editing permissions on some (e.g the Members, Contributors, etc) and adding new site groups according to your clients requirements. After this step Area Administrators (or others) may not be able to add all web parts, they see only a subset of all available web parts. Why ? The cause is web part gallery permissions need to be re-set (or re-inherited).
The KB Article is here – http://support.microsoft.com/default.aspx?scid=kb;EN-US;888840