-Quit the Mac App Store
-Launch the Terminal (located at /Applications/Utilities/)
-Type the following command into the Terminal and then hit return:defaults write com.apple.appstore ShowDebugMenu -bool true
-Relaunch the Mac App Store, the Debug menu is next to “Help” in the menubar
-Select "clear cookies" in the debug menu first, and second to select "reset Application".
This method doesn't work for me.