as I understand it, skimmers need a constant level of sh!te to be skimming constantly, once they have taken all the cr@p out, there's nothing left to remove, until it builds up again.
this is why an underskimmed tank will constantly produce a level of skimmate, but an overskimmed tank wont
another reason? you have your skimmer in a sump which doesn't have a constant water-level, and the level varies in the contact chamber.