I have seen two types of problems with those buttons:
-"soft" one, the contacts are dirty (pressing the switch feels the same). In this case just use some contact spray, it works great. You don't even need to open the switch.
-"broken", the 'membrane' has been completely broken. You need to replace it. Those switches can be found in almost ANY electronics store, but you need the right height. If you don't care too much about looks and don't have new switch of same height, open it completely and replace membrane. (saves you from soldering). try to leave one of those 4 plastic holders 'alive'..
