Iphone charger doesn't charge every iphone?

My friend has a 30 pin iPhone cord for her iPhone 4.It can be used to charge her iPhone fine but when I connect my iphone4s to hers it doesn't charge anymore, it used to. Why does it only work on hers? How is that even possible?

Pins on either the charger or one of the phones are bent. As such, proper contact between the charger and one phone is not working right.