Even conventional Al Qaeda attacks have been botched because of the incompetence of their perpetrators.
A few we know about: World Trade Center 1993
The Bojinka Plot
The Millennium Plot
The New York City Landmarks Plot
Richard Reid
Jose Padilla, a borderline retard, was in charge of Al Qaeda's dirty bomb planning. Enough said.
In my opinion, the highly complex attacks of 9/11 could have only been successful with material assistance from the highest levels of a rogue state, most likely Iraq. Without the same from an atomic country (which I think is almost impossible, even from Pakistan or Iran), I think nuclear weapons are far beyond Al Qaeda's capabilities.