With the surface skimmer it will be sensitive to water level. Because as you pull water out of the tank the water will get blow the bottom of the top finger strainers on the skimmer box. When water cant enter the skimmer overflow box it cant be pumped into the skimmer body.
The skimmer box reinforces you to maintain proper water levels in your tank to keep the water level above the bottom of the skimmer strainer fingers.
can be caused by a list of things and some of them would have no effect on the amound of skim produced.
Rase and lower your skimmer cup if you wan tit to produce more skim. The lower you make the cup the more watered down the skim will be.