Had the same issue. Sony (1-800-345-SONY) basically told be to pound sand. Good luck though. I spent a little over a month fighting, and finally got a resolution from Sony. It took patience and perseverance. A lot of emails, phone calls, posts, complaints, badgering from fellow forum members, etc. I wish you luck, but the outlook isn't good.
Sony doesn't care to monitor who used the card. They see it simply as the funds were used. They received their money when they sold it to the retailers, so where it goes from there is not their concern. Customer relations are the pitts in this regard. Bunch of sods.
If you're from the UK, you might want to post your issue in Website and Forum Help, Feedback & Advice.
Also, sometimes when we purchase a prepaid card, the person at the register forgets to activate the card. If you still have the receipt, I suggest taking your card back to get it scanned.
Lesson learned: Do not throw out the receipt until after the card's code has been inputted by you and accepted by the PSN.
Definitely hold on to everything until your funds have been accepted and spent. I had everything, scanned it and sent it to Sony, and I still had to fight to get my funds. You are probably better off trying to get the store to give you a new one. Sony does not want to help anyone with these issues. They either don't believe you, or they don't care about you. That is my experience.