Hans Blix doesn't want to offend the Iraqis by taking them by surprise:

"One must remember always that Iraq is not a country under occupation," he told the media. "You cannot go on forever to take the authorities by surprise. Inspectors are not an army, not a commando troop that can leap in and shoot their way to the target."

Okay, if the lack of a military occupation of Iraq prevents the use of surprise inspections to do a thorough job of weapons inspection then one obvious solution presents itself: Invade.