Memory card help

I have a ton of playstation one games. We had an original playstation 2 and my BIL broke it. So he bought us a playstation 2 slim as a replacement. When we put in a playstation one game and try to save it, it says there is no memory card in the slot...but there is. these are the same cards that we used with the other unit. They worked fine. When you put in a playstation 2 game, it says the memory card is there. Any ideas???

Thanks

Danica

I did do a search on the board but could not find anything similar to my problem.