Night Service doesn't run every night

Apparently the GreenHopper Night Service is not running every night, as expected. Customer's get frustrated because their rankings appear wrong in the boards. Normally clicking at the "Optimize(repair)" corrects the ranking, but the question is, how often you should click at this button ? Why only using the "Project Overview" mode works for ranking issues ?
The night service should run every night so users don't need to manually press the optimize button, but this is not working always.