One of the interesting aspects of the PDB is that it did state that OBL might make attacks against the US. Now consider that there was also a lot of other intelligence suggesting that OBL might use airliners.

Yet still no one has explained why, after hijackings actually DID take place on 9/11, that no fighter planes were scrambled, when that is the normal practice.

You would think, after intelligence regarding POSSIBLE hijackings, that when they had an ACTUAL hijacking, there would/should have been an immediate military response, even if they didn't know precisely what type of hijacking it was. Hell, they had scrambled military fighters when small private aircraft went off course -- why not against major airliners on 9/11?