What happens if you press the middle part all the way down and slide your finger towards you slowly?
If you open the switch you might be able to figure it out. My guess is that by bending the leaf attached to the switchplate to have a slightly sharper 'kink' in it, you’ll be able to fix it. It has probably just slowly bent into a straighter-than-original shape with age and use. As a result, when the slider goes straight up and down it doesn’t sufficiently push the bump in that actuator leaf into the little metal foil contacts.
report back after you've broken it in...
10,000 key presses per affected key...
ready. set. go...Show Image(http://a.deviantart.net/avatars/g/a/gameaddictplz.gif?1)