First, what Mac computer is this for? Does it meet the system requirements for Mavericks? OS X Mavericks: System Requirements
Do you get an error message when it fails.
I upgraded an iMac last week and any time I tried to use the Mavericks link on the "Updates" page of the app store, it errored. Called AppleCare--they couldnt figure it our. The error I got was, "The server gave an eror during download 503: Service not available.
My workabout: I left the App Store Updates page and returned the the App Store home page. On teh right edge is a list of "quick links":

Using the quick link for Mavericks took me to a regular App display page and I was able to get Mav downloaded and installed without further issue.