Like for Valjoux 72 and Venus 178, the hour subdial is probably driven by the mainspring directly and not thru the train wheels (which drive the second and minute chrono hands). There is a the lever that is kept in a certain position by a spring which blocks the hour chrono wheel. It is released when you press the start button. Most likely in your case that lever is not being activated when pressing the stop button.
It is usually an easy problem to fix that only requires removing the dial/hands and working on the dial side of the movement without full disassembly. However, you might have a broken spring.
A watchmaker that knows vintage chronographes should take a look at it.
