Just bought a PSN Redeemed prepaid card a 2 weeks ago and I put it in it worked no problems, but then I bought another one for this week I put in the 12 digits in it said it was incorrect or not valid. Stlil have the reciept and it says it was activated. With that said I entered it in more then 5 times and still came out short. I read it was case sensitive so I put it in slowly and carefully and still nothing so I went to qriocity.com to put it in, still it says it was incorrect or not valid anymore. Seriously don't know what to do.
Someone else posted a similar issue earlier today in another forum. It may be a problem on Sony's side, although there haven't been enough reports to indicate that the entire redemption system is down. I suggest that you call SCEA customer service at 800-345-SONY.
gkpama is correct, there have been a numbe rof people complaining of the same issue all day today, trying both directly on their PS3's or through the Qriocity website, I note from your post that you have tried at leat 5 times to input the code, just so you know Sony does lock some cards out after multiple tries to prevent fraud, just thought I'd mention it.
A friend of mine had the same problem. She contacted CS and they gave here a new pin.
I would try calling CS (hit the "contact us" link at bottom of this page) and make sure you have your receipt and card on hand.