Lots of people have reported having to install that thing more than once.
When you installed it, after reboot, did you absolutely get a window saying it was successful? (for me, the window took 45-60 seconds to pop up, and it failed twice before it worked (didn't want to install on battery)).
Try reloading the updates section of the Mac App Store again, and again.
Happened to me, too. With the exception I have FileVault enabled. Contacted Apple Support and looking forward to resolve this issue. Otherwise I'm stuck with Mountain Lion. Can anybody confirm this has something to do with FileVault, actually?
When you installed it, after reboot, did you absolutely get a window saying it was successful? (for me, the window took 45-60 seconds to pop up, and it failed twice before it worked (didn't want to install on battery)).
Try reloading the updates section of the Mac App Store again, and again.