I have the G3, sitting in exactly 10" of water, w/ the recir mod, and the gatevalve is VERY close to closed to get it skimming properly. You may also want to set up some type of ball valve on your input so you can adjust the amount of water going into the skimmer. I have a 1" over flow that splits at a "Y", one side goes directly into the skimmer, and the other just dumps into the sump. I have a ball valve on the pipe that bypasses the skimmer, so when I close it a little more water enters the skimmer.

You can also try raising the gate valve up and down to see if that helps, but I would think you would need between 11"-13" of water for a skimmer that tall. Good luck.