I cleaned it with hot water and a towel before I put it in the sump. Right now it will only dump skimmate in the cup if the water level is at the very top of the chamber/right before the neck. When adjusting the water level, how much will make a difference? Are we talking a few inches from the neck?

Also, is it common to get gunk caught up in the neck of the skimmer? When I pulled the lid off, I see small black/brown globs lining the inside of the neck.