Replaced key fob battery and shell it does not work

I changed the battery in my key fob because the signal seemed weak when I would try to lock or unlock my doors. After I replaced the battery and the key fob shell with a new one it will not work. I tried to put the new battery in the old shell and it will not work. I was told my the dealership that the fob is still programed to my car they are not sure why it is not working. Please help..