We have quite a few DL in Exchange and we are finding that members that we know for a fact were in there are no missing. Has anyone else experienced this?

I don't suppose you have Restricted Groups applied in Active Directory? http://technet.microsoft.com/en-us/library/cc785631(WS.10).aspx Oliver Moazzezi | Exchange MVP, MCSA:M, MCITP:Exchange 2010, BA (Hons) Anim | http://www.exchange2010.com | http://www.cobweb.com | http://twitter.com/OliverMoazzezi

Yes we do have them set up.