If you have screws that repeatedly work loose:
An okay fix: Put a drop of removable "thread lock" such as Loctite on the threads. (Make sure the stuff is the removable type! If you use the "permanent" stuff, your screw is screwed.)
A better fix: Wrap the screw threads with a layer or two of Teflon tape.
The Loctite method is messier, may gum up the screw and its tube, and makes removing the screw later a bit more difficult, but it does work well.
The tape method is cleaner, reversible with little effort, and you can repeat it without having to clean the threads of old thread locks.
Over and out.