I had one and I kept it higher, I never saw it foam but if I didn't look at it for a week, then there would be green ooze in the bottom after a few days.
My mom keeps her's higher, but he skimmate it practically water, so it's more top off for her, and I don't think it's getting so much protein out, as just discolored saltwater.
I think it take some fiddling with to get it right... took me a few weeks to get it down. And also, if you tank is fairly new, it make not have a whole lot to skim out, especially if your bioload is fairly light.
What size tank, and what/ how many fish?