Perhaps they are not accepting the initial card you get at the store when you first sign up.  You have to wait a couple weeks til you get the plastic card in the mail with the account number on it.  Then you can use it everywhere...

Check out some threads on the GD or Newbie board.  I got bashed pretty hard over there yesterday because I claimed hardly any cards work.  Greendot was one that was claimed to work, BTW.

Good luck with your search....I gave up and used a personal card.