Problems with memory on Fuji Fine Pix S5600

Hi. I am a technician at Hugh Baird College, Merseyside. I have 4 Fuji Fine Pix S5600's which i give out to students on a daily basis. For some reason the memory space on the cameras is dissapating. The worst case is on one camera where the mem card is showing as full but there are no images on the camera to remove. this is not a problem with mem card as when you change the card for an empty one, it still shows as full. On the others, there is only space for 6 new images on clear mem cards with 512MB (normal sized images). not sure what else i can do! any suggestions? cheers!

Brychan Tudor