I have an MRC MR2 Protien skimmer. I just can't seem to get it to skim properly. The water level inside the column constantly fluctuates and the density of the air bubbles in the column seems extremely week. When the bubble density appears very week, I clean the becketts and expect them to be dirty, but they are not. When I hold my finger next to the air inlet valve, it seems like there is no suction. I had the 18" column extension on the unit but took it off thinking things would improve, but they did not. The pump I'm using is a Reeflo Marlin pump which I think should provide more than enough flow. What could be the problem?