I had this problem in my first tank, when I didn't have an overflow box. I tried to get the skimmer to draw water from the surface but I could never keep the level of the tank stable enough to have it work consistently without constant adjusting.
If the tank level was a bit high...the skimmer would draw from below the surface.
If the tank level got too low...the skimmer would run dry really quickly.
So I ended up directing the outflow from the skimmer to FALL into the tank. This would break up the skum and suck it back down into the tank.