In my case I accidentally heavly dosed Kalk for 24 hours. While nothing was harmed in my tank, it was plenty enought to clog this elbow. One day my skimmer was working, and the next it wasn't. But a quick soak of the elbow in vinegar took care of this.
Another tip on adjusting the 9002. Only make one or two very small adjustments a day. Make these adjustments on what is getting collected in the cup, don't even worry about how high the foam column is. I find with mine, the smallest turn, like less then 1/10 is all that is needed to adjust it. If I turn it 1/4 or even 1/8, the skimmer REALLY changes how its skimming. Mine at least seems to have a very touchy air adjustment.
Now to adjust, I check the skimmate, turn the knob 1/10 depending on whether it's too wet or dry, and then walk away. I don't even look at my skimmer until the next day. The only time I might make a faster adjustment is when I turn the knob to far and it quickly fills the cup with clear water. :eek2:
Once I get it set, I don't have to adjust the screw at all. It runs very stable for several weeks. It seems the only time I have to adjust it again, is when I change the water level to much when refilling after doing a water change.
Hope this works for you!
Jeff