Oh it's quite possible they screwed it up. Look at Ramzi Youssef setting his flat on fire in the Phillipines trying to build a bomb to assassinate Pope John Paul II.
All the experts are in Iraq fighting the real experts, the US military. Even the 7/7 guys were rank amateurs who managed to do minimal damage given the circumstances. In fact, when looked at objectively, 911 was by far the most successful operation conducted by islamists, and they didn't envision the towers collapsing. That was an incredible stroke of fortune for them. They are cunning and vicious, but their tactics and weaponry are something out of the 14th century. They are like some tribe from the land that time forgot.