The most common problem people have had has been because of the license servers from esellerate not responding in a timely fashion. Basically, I've had people tell me in tech support that they tried literally dozens of times before success. Just in case persistence doesn't help, you should check out this article:
Activation Problems
If you still can't get it to work, you might want to try activating late at night when fewer people will be trying to activate or you can contact us in tech support - see my sig.