Perhaps just give it time to see if will get added to your account, sometimes it takes a bit of time (hour tops I would think). After that, check the TapJoy wall again and see if the offer was removed, if it was and you still didn't get your plat, then perhaps contact STS (I don't think they work with those kinds of issues, but I could be wrong). I know that sometimes you need to have certain criteria met in order to get your plat; for instance, there is an offer right now (Buddy Rush) that requires you to log into Facebook before you can get your plat. So if you don't have a Facebook account, then you cannot fully complete this offer. There are a few others that have required you to play the game up to a certain level before you get the plat.

Side note: I have never been able to get Storm 8 games to give me plat (but the offers don't remove from the TJ wall either), could just be me but I gave up on those few games.