Problems with Casio camera Exilim EX-Z1000.. Help please..?

Hi! I'm using an 8 GB memory card with my casio exilim EX-Z1000 camera. I transferred all the pictures from my memory card to my laptop and deleted most of them. But when I put the memory card back to my camera, it said that I could only take 40 pictures. I tried with another 8 GB memory card and I got the same problem again. That is, I can take much less pictures than I normally can. Thanks for your help.