They couldn't reboard the original flight because it had left before they were cleared by the FBI. An AirTran spokesperson claimed that the problem with rebooking on a later flight was that the family's data hadn't been updated in the computer file available to the gate agents. While this smells of blaming "the faceless nobody in the back room," it is well known that the airlines' chief weapon against their customers is the withholding of accurate information. In my experience the gate agents are frequently victim of such information control.