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I recommend checking activity monitor, there you can find out if it's indeed actually downloading or not.

1) Check network and see if App Store is using significant amounts of data, if so then yes it's downloading and you should just wait it out until the data stops coming in.

2) You can also check the files that the App Store has open and one of them should be the Mavericks Installer (possibly a temp file). You do this by selecting the App Store process and then get more info (cmd + i or press the little i button in the toolbar).



If it really isn't doing anything here's three workarounds:

1) View your purchases, select mavericks option-click the "Installed" or "Downloaded" button.

2) Hold down the option key and click the Purchased icon at the top of the App Store window, there's a good chance that the status button for the app in question will change from "Downloaded" to "Download," or from "Installed" to "Install."

3) Create a new user account, log into that account, download and install mavericks on the new account. Once done, delete the new user and return to your normal account.


Good luck.
 
I faced this too. It was simply showing 'Download' option. But when I checked the it in launchpad, Mavericks was downloading. The Appstore isn't showing the download status, so it is actually download. Let it be for sometime and then check the bar under the Mavericks Icon on the dock.
 
Maybe it did download and you're just not looking. Look in your applications folder and see if its there or not.
 
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