I have had the same skimmer as you for about 2 years and i can say its construction and operation is first class. However, it took a while to set up ( 3 days )
In order to set this skimmer up;
Turn the air pump off.
Leave the re-circ pump on.
Open the exhaust water valve fully open.
Turn on your supply pump.
Gently close the exhaust valve until the water level in the skimmer is about 1.5 inches
below the narrow bolted flange joint.
Turn on the air pump.
Wait and watch to see that you do not have a continuos overflow,if so slowly open the exhaust valve accordingly to stall the overflow.
This operation assumes that you have not altered or restricted the flow through the exaust pipe other than closing the exhaust valve as above.
Even the sump end of the exaust pipe being underwater will have a negative affect on
the skimmers performance.
I have found its optimum max flow through should be about 5500 lph
Good luck.
p.s i dropped a cleaning sponge in the skimmer once and it didnt work for 2 days !!