Don't know what brand of hourmeter you have, and they do differ in design. But, I've had two that were the old "odometer" style, with a group of numbered wheels to show the hours. These were driven by a mechanical "pecker" that jabbed at an escapement that in turn drove the first wheel
, which drove the second one... At any rate, after a long period of service
, they simply wear out mechanically, and AFAiK can not be repaired. My solution was to buy a new aftermarket meter in both cases. Not very expensive, but do remember to note the hours on the old one for adding to the new total (when you are feeling honest!).
Wish I had better news for you, mate!