What is your biggest GM frustration?

What is your biggest GM frustration? 

When prepping or running a campaign what is one of the most difficult challenges you face? What are some resources that would help you at the table?

As I create RPG resources I mainly focus on what I need at the table, but I would like to hear from you. Just reply to this email with your biggest game master frustration.

Keep Gaming!
Matt
www.dicegeeks.com