Having lived with some pretty cheap instruments, I don't doubt that some tuning machines may creep. You can get replacement gears for some kinds of machines, but unless you start with a quality piece repair may be less effective than just replacement.
A couple of things that may help you to "live with" a bothersome tuner:
Uneven winding of the string on the tuner shaft can cause a "lumpy" winding that just refuses to stay in particular positions. Try to avoid winding a lot of string onto the shaft, and keep the winding as smooth/even as possible.
Try to always tune "up" to pitch. If a string is sharp, turning it just down to pitch leaves any "slack" in the machine "open." Turn it down to below pitch and then bring it back up, so that the gears are "loaded" in the same direction they "hold" to keep the string up.
A "sticky" nut can cause a difference in string tension above and below the nut which, when (not if) disturbed, can redistribute to detune the string. Take a look at the nut to see that the string notches aren't worn deep enough to let the groove "grip" the string. A very small bit of powdered graphite just in the groove sometimes helps. (I knew a fellow who played for years with a small slip of cigaret paper under each string in the nut, because he claimed it kept the grooves "lubricated." I'm not sure what he added for "lubricant" but it often looked like ear wax.)
Best advice is to get a trusted luthier to look at your setup to tell you what needs to be done.