Sleep mode problems

I am having problems with my ibook and the sleep mode, it isn't like most people's problems that it doesn't wake up, but my problem is that it doesn't go to sleep anymore when i close the lid. It was working just fine since i have had the laptop, and one day about a week ago it just stopped going to sleep when i close the lid, but it goes to sleep when i tell it to through the apple menu, and it wakes up just fine when i need it to. Does anyone have any suggestions, or maybe have heard of this before. thanks

maybe there is a fault with the sensor that detects whether or not the screen is closed?