Skimmer production is dependant on organic load in water and how well it is designed.
With out knowing tank size, bioload, skimmer design and setup there is nothing to base an answer on.
Kind of like asking why is my car not fast without knowing what it is, sorry
Do you have a marine tank or fresh water tank?
I thought I saw where you had a 40 gallon salt water tank, but you're posting in a fresh water forum?
But if it is a marine tank, then as Doug said, we need a little more info. Even a top notch whiz bang skimmer will only make foam if'n there's proteins to be removed. It could be you are still way to clean to get any scum to remove.