Signzzz.... it will usually produce skimmate when i introduce some new things in but the skimmate usually last only for a day.... subsequently, foam column and collection cup is spotless clean.. Have a feeling that the skimmer is just acting as an air pump now to improve dissolve oxygen level...
Just introduce a PT in yesterday and a YT in the next few days... If still no skimmate, think may need to change skimmer.. haha...
As hammy put it, if the NO3 remains to be zero and no diatom problem in the tank, why bothered if the skimmer is foaming or not!!