Here's where the plan went wrong for Flight 93, meaning that WTC7 was left without a plane in it
Wikipedia - United Airlines Flight 93
"The aircraft was scheduled to depart at 08:00 and pushed back from gate A17 at 08:01.[26] It remained delayed on the ground and did not take off until 08:42 because of heavy airport congestion, just four minutes before American Airlines Flight 11 crashed into the North Tower of the World Trade Center.[27] The three other hijacked flights all departed within fifteen minutes of their scheduled times."
http://en.wikipedia.org/wiki/United_Airlines_Flight_93