I am trying to download and install Mavericks and am getting nowhere. When I go to the App Store it is marked as "Free Upgrade". So I click the button and, after logging in, it changes to Install. I click that and it changes colour and is disabled for a few moments and then changes back to "Install". If I exit the store and re-enter the button is back to "Free Upgrade". My computer definitely meets the requirements so what am I missing? Note that I am a total newbie to Mac so it could be a real simple error on my part.