Is this a new movement or one you have cleaned? does it just stop with power on. If the movement doesn't start up when you start to wind it might be out of beat. What does it sound like when going, does it have an uneven sound? if so it is out of beat and needs adjusting. If you could post a photo showing the balance I can advise what you need to do.
In order for an automatic to run for longer periods or to its full potential it needs to start with a full wind. Not sure what the watch went through in the last 10 years but it sounds like the movement is operating as it should. It could have been in the last 10 years it started with a full wind and he consistently wore it for 10 years? Or did he ever manually wind it? Did he always wear it when sleeping? If so how many times in 10 years? These questions may not have answers but could explain the difference. Movement is def a big part of the winding process in an auto watch, it is within it self a form of winding, which is why they sell automatic winding watch cases. So by not manually winding and not wearing it while sleeping and purposely leaving sitting for long periods to see what the reserve is will all play a part in this.