Cable tension moves it up, slack moves it down.

Somehow, you are losing the tension the shifter creates.

Easy
-Loose bolt
-broken cable
- housing slipped off the braze on

Medium
- cable frayed or rusted inside housing
- housing ends are blown out
- snapped deraileur hanger

Hard
- mechanicals inside shifter are stripped


There's lots of other stuff that can cause bad shifts but not too much else that can cause no shifting at all. I

I were to guess, based on extremely low info and no pics, I'd say the housing is slipped through the braze on where you thought a barrel adjuster ought to be.