Since those get the vibration of the strings through contact with the instrument to which they're clamped, check to see if the spring that maintains that contact isn't getting weak, or if there's dirt or crud between the wood of the instrument and the tuner's clamp/pickup surface.

I don't know that the actual mechanism contains anything that can weaken and fail -- but I don't know what the innards of those things involve, so perhaps the guts include components that can, indeed, tire out or fail.


Bob