The car/fan belt (only 1 belt) keeps coming off. Why? on 1996 Mercedes-Benz C220

Hello. Again, I have a 1996 mercedes benz C220. It has 250,000 miles on it. And its fan belt keeps coming off. I thought it might have been the belt; so I purchased a new one. And then I put it back on; but it came off again within few seconds. Can someone tell me how I can fix this? Please? Thank you very much.

Asked by for the 1996 Mercedes-Benz C220