pu selector switch for what kinda geetar? Strat, LP, tele, RG, or any other models? Gotta be more specific...
anyway, if its for a strat, the selector switch only got 2 holes and these 2 holes are threaded. The more turns as you turn the screws, it shud secure the selector switch tightly. If its loose, that means it aint secured properly. It cud eventually be dislodged
anyway, we are just dealing with a part with 2 holes, 2 screws and a pickguard, dont be stressed yet.
Worst come to worst, removed all strings, removed pickguard, look at the selector switch, unscrew the 2 screws that secure it, screw it back again, adjust and see how it goes. Usually it cud be
1) obstruction (removed obstruction and fix back the selector switch)
2) the treads on the screws are gone(get new screw and replaced old ones)
3) the threads on the selector switch holes smoothen out(find another screw and "re thread" the hole again, use some force)
where to find the screw? Heh, either hardware shops or proper geetar shops that sell those. Worst come to worst, get a new selector switch(it comes with the 2 screws usually) and replace the old one, but this way much more expensive...