problems with overflow

so i'm having some problems with my overflow system.. for some reason, it is pulling so little water into the sump that the pump is barely pumping back any water..

does it have anything to do with the 'inside the tank' part of the overflow box being adjusted to high/low? my pump has a valve on it, so i can adjust more/less as i need too to keep the water flow even.

also not sure if it matters, but figured i'd ask, on the siphon tube on the overflow box, should the lower end be on the inside or outside of the tank? and should there be any air inside of it? i can't seem to keep it all out.. also, does it matter that the box leans back against the outside of tank, or should it rest even on the trim of the tank?

i'm still a little fresh on this sump filtration stuff, and am not made of money, so anything you could throw at me would be greatly appreciated.. yes lots of questions - i just dont seem to be getting enough output to give it ANY worth a damn water flow from it - this powerhead stuff sucks! thanks guys.

i do have a venturi valve, should i just place it on the top of the siphon tube? (edit:nvm found that out!)

well i have another pump but how do i know that it will be the same amount of water flow as what's being drawn from the overflow?

and i have the basic overflow box system you see everywhere. so it has the pool tubing, with no sharp bends..

here's the system type i am running.. how should i know how high/low the left box should be adjusted?

also, my siphon tube is lower on one side, so which box should the lower side be in, if it even matters..