When the customer says the bin isn't filling up overnight like it always did usually isn't based on any hard facts, just a simple observation that will lead you nowhere.
You have to weigh the amount of ice produced for a short, but fixed period of time (like 30 minutes) and extrapolate that information into lbs/24 hr. Compare that with what the specs say the machine should be producing at those conditions and you'll have a much better idea of where you really are.
In your case I would do the production check on each machine separately.