Unless the filter floss is clogged it shouldn't affect the water level too much, so I would leave that alone. Where do you have the skimmer gate currently? I have the skimmer gate just at the bottom of the top mark.
It could be that you just don't have enough water in the tank, it's hard to get the right amount of water, with the skimmer gate in the right place, the water level in the DT doesn't show below the rim, so that the skimmer works effectively. Its trial and error unfortunately.