Physics question about the lift of aquarium pumps

We’ve an external aquarium filter that works when the intake is a couple of inches below the surface but don’t work with the intake extensions that reach the bottom of the aquarium about 24 inches deeper. (The extensions are clean and work on other similar pumps.) See picture of setup HF0300 about third to half way down this page Aquarium Fish Tank Hang On Filter Pump Hang-on Bio Filter 290L/H HF-0300 |

To this amateur it would seem that the lift should be from the surface of the water and the extra two feet has nothing to do with it. What am I missing?

You are right - it should work no matter the length of the dip tube. I’ve used many of these filters, and they all worked fine with long dip tubes. So, there’s something else wrong. I’d suspect the impeller.