I'm having the same issue as the original poster. I have a 16ft frame pool with the intex swg (1200 gph) with cartridge filter . The skimmer is a bit finnicky.
I happened to notice that the intake was indeed clogged up underneath the blue screw-on adapter used by the skimmer. We have cottonwoods that make an awful mess this time of year. I cleaned that off and it seemed to improve, but didn't go away.
Air is being pulled in somewhere, it's obvious from the sound of the pump. If I pull off the skimmer hose from the blue adapter and it just pulls water through normally, it sounds absolutely fine. I add the skimmer hose back and the pumps returns to it's labored sound.
I've tried adjusting the skimmer in every which way. The hose is very stiff and causes the skimmer to never quite sit level, but even if I hold the skimmer in the most proper position it still draws in air.
This time of year the skimmer is a big part of keeping the surface clean but most of the time I just have to turn off the skimmer.
If anyone has any new suggestions that would be great!