Ipod Charger not working for iphone 4s?

My ipod charger charges just fine with my ipod 3g
but when i plug in my iphone 4s into my ipod charger, it won't charge?

When you plug the cord into the PHONE, does it slip in easily?

If the answer is YES, then that's the problem. The springy action on the sides of the plug that goes into your phone has lost it's firmness. If you plug in a new charger, it will work.

This happened to me, and it worked.