Why are my iPod apps disappearing?

Hi. I've had my iPod since xmas 2010, and I've just downloaded a few games. Then, once I'd quitted them - on my homepage, the Facetime and calender app are missing. They still have the shadow and the name and you can still go on them but the pic is missing. Then, when I flick from home page to the second page and back again, the settings will be missing. It's a bit odd and wondered if anyone else was experiencing this and if anyone knows how to fix it?