Adjusting Deltec skimmer

[TiggrReefer]

I have a Deltec skimmer that is outside of my sump. It’s been great for years, but recently I’m having a tough time getting it tuned. I can’t seem to get much skimmate. What kind of maintenance should I be doing ?
I used to just fiddle with the air valve and/or water inflow valve.
TYIA

 

[lapin]

The only thing I clean is my cup. I cleaned the tapered part 1 time and had to mess with it for weeks before it settled down. Lesson learned.