Two scenarios seem most likely to me. One, the stuff was well hidden to keep it away from Blix. Then one of two things happened; either our advance was so fast there was no time to take it out of storage and distribute it, or it was destroyed by someone who didn't want to be tried for war crimes. Second, the stuff was moved to Syria or Iran.
IMHO we are unlikely to find anything through our own inspectors, but will have to find the truth through interrogation of senior officials.