the biggest issue is often having a skimmer too large to perform well. If the neck is large relative to the tank volume, it takes a lot of organics to build a nice foam head. The concentration may be high in the tank, but if the water volume is too small for the skimmer size, the total amount of ogranics may not be sifficient to maintain a nice foam. This is why many people who oversize their skimmer end up disappointed in performance, even if the skimmer is a well designed model.