I have a MTB equipped with Shimano brake combo rapid-fire shifters. My front shifter won't shift. When I push the lever to change to higher gear, it comes back to initial position. The gear changes as long as I have kept the lever pushed, but that's not right. When I push the lever, the mechanism inside just won't lock in place. Comes back to initial gear.
My first suggestion is some sort of cleaning and lubricating solution. Here we use Liquid Wrench, 3-in1 or Release All. Spray a bit in and then work the shifter a bit. Let it sit a few minutes and then perhaps see whats changed. Try again if needed.
If that doesn't work boiling water can be used as an alternative solution.
If the mechanism is still failing after youve tried those you may need a new shifter. Replacing pieces inside just isnt advisable.