How are you using the skimmer? Is it plumbed or in a sump? If it is in a sump, then I recommend elevating it out of the water as much as possible. If you can keep a constant water level it tends to operate better. Also use a larger than recommended pump then the water height will not matter nearly as much. When I went with a larger pump, I had to restrict the flow of air into the downdraft chamber so as not to have too big of a bubble being produced.
The gate valve adjusts the water/foam level in the column. This adjusts how much is skimmer and how wet/dry the skimmate is. Make small adjustments to the valve (1/4 turn) and wait a day and see how it skims. Raising the water level will get you more (wetter) skimmate. Lowering the water level will get you less (drier) skimmate.