apple software update does not work

Apple Software Update presents software with updates available (currently Airport Utility, iCloud Control Panel, and iTunes). I click "install 3 items" and accept the license agreements and say "yes" to User Account Control. It always just takes me back to the Software Update screen without updating anything. This has been going on for several months, and it doesn't matter whether I request updates from within iTunes or get the periodic update screen from Apple. I'm currently at iTunes 10.6.1.7 and it's offering updates to 11.0.5.