Something I’ve been learning, while developing my Tilapia tank poop sucker… The flow ( based on the diameter of the hose ) has to be right for the diameter and length of suction tube or it won’t pull solids up and out of the tank… a bigger diameter suction tube pulls lots of broken down waste out, but generally heavier solids won’t go up the tube ( which is kind of the idea, or you would just suck up all your gravel ) you can find a gravel vacuum with a little smaller suction tube might pull out the solids better, but if your substrate is too lightweight, it will want to suck that out as well. Also I’ve added elbows where my hose usually bends as it comes out of the tank, a kink in the hose there, reduces the flow, thus limiting the size of solids that will vacuum out…
As your tank matures you may find out the waste breaks down better, and is easier to remove.
Also the kind of foods used can also effect the solidness and density of the fish wastes, so depending, you may even try changing food’s your fish are eating