I dunno if this was posted already here, but I'm going to provide a logical reason why your free platinum did not take.

Sometimes in PL certain apps will appear on the 'Tapjoy, Free Platinum!' menu more than once. If you already have received your free platinum for a specific app, that action is remembered by Tapjoy.
So if the same app reappears on the list a couple weeks later, and you download it - you will not be eligible for free platinum.

Sometimes its hard to remember which apps we downloaded and such, so sometimes it can come as a surprise and leave you thinking..."wtf?"