Mm. I just lost my mobile phone, the ds, the psp, and all my other good games, and the hard-drive with all the downloaded games on crashed. So I bought angry birds too!
No, seriously, just write Sony support. Might be an ID mismatch between the dlc and the game. They'll be able to help you pinpoint the problem, and if it's specific to your system/store combination, or if it's something else. They'll want to know exact information anyway, so write your local support.
:/ It's just about checking that the ID on the dlc package you downloaded is the same as the full game you bought. Sony could do that for you, and help you figure out which package you would need to buy.
Then again, I guess they also could choose to not give a **bleep**.