The calabration gauge or "keys" got wore out? I've broke a couple but never wore one out, either way I have spares in the tool box. Since they are made in a stair-step manner, each step has a number, if the bag says calibrate to #14 to apply proper amount of product, #14 gets placed into the spreader and it's set.
If someone pushes the gauge further in than instructed, well I don't know what to say, maybe they need a new profession,
it can't get any easier than this. I would surly expect if someone could not get the simple calibration done correctly, how in the world could they get more difficult procedures as described above done?
I agree the spreader can get out of calibration due to knocking around, I always try to monitor if the proper amount of product is be applied, so far no problems. If in doubt, take 15 seconds and check calibration