Turboflotor Multi SL Hang-on, Not producing skimmate when return pump is on??

[jrcastro]

I just bought one used & installed one in my 18gallon sump. Question is when i ran the skimmer with the water inside the sump only it produced lots of skimmate in the matter of minutes. But when i started the overflow & return pump on, it stopped producing the skimmate. My return pump is approx. 600GPH at the tank. It seems like the skimmer is just mixing water in the skimmer, there are bubbles in there but i guess not enough to make it rise.
You can't adjust anything since these are supposedly just plug & play. Any ideas anyone?
thanks in advance

[jrcastro]

I forgot to mention there are no valves to adjust the output of the return water. Could the flow of the return pump have an effect? or could my tank water be very clean?
I dont think so since its a new setup & im curing LR.