For the last 20 yrs I have refused to sell solution feeders because of this and other inherent problems. I think the 8' is too high but, in that plastic fitting on the water line, there is a check ball valve, they stick preventing the solution pump from being able to inject solution through it.
So you have to remove and clean it if possible, or replace it with a new one after shutting off your water and bleeding off the pressure.
It kinda sounds as if you don't have a retention tank, or the plumbing wouldn't be up at the 8' ceiling.
There are check valves in the pump too and you should be able to buy a rebuild kit for you brand feeder (like Chem Tech is it). Or your style of injection system doesn't have any because it squeezes the tubing to move the solution.