Hmm, interesting. I'll have to watch for that effect. It could be happening for me. But it's hard to say. Usually the skimmer works better after I've emptied and cleaned the cup, which could be explained by the crud preventing the foam from spilling over, but then I usually give the venturi a quick clean as well at the same time so it's hard to say, maybe the better initial performance is because of that instead.
Interestingly this last week I've gotten a half cup full of skimmate. It hasn't performed so well in months. Maybe it's heard me talking about the mods I'm going to make on it and it's trying to shape up
Honestly I do find that the venturi is very sensitive. Just a quick poke through with a nail is all I do at cleaning time, takes basically 10 seconds and it ensures there is no blockage starting. Give that a try maybe?