Why does my skimmer do this?

[SnRFishies]

Ok I have a strange question. When i have a filter sock which I change every 3 days on, my skimmer collects a lot of skimmate real dark and I feel its working.
But, when I take it off ill let it run for a whole week even 2 weeks once and i barely get any skimmate at all. And my tank have enough bioload where it should skim a lot. Coyld my flow through the sump be too fast.

75 gal display
20L sump
Mag9.5 return
Bubble magus nac7

 

[ZR1001]

Does the water level in your sump change when you remove the filter sock?

 

[SnRFishies]

Does the water level in your sump change when you remove the filter sock?

It does change very slightly though. It drops about a quarter inch or so in the return section