Yes, the fobs do wear. Sometimes if you open them up and put a bit of card on top of the battery before re-assembling it will work better.
If opening the key make sure the immobiliser chip does not fall out!!
You can also get hold of a fob which is in better condition than yours and swap the casing, which is what I did (found one in a car in the scrapyard!).
If you need a complete new remote it will have to be coded to the car using diagnostics. Despite what many ebay sellers will try to tell you, a key which is not coded to your car can not be manually set using the "ignition on and push the button" method.
vaux
November 2010