Liquid wants to find equilibrium, that's just basic physics, so as long as your pump is below your volume source, the pump head should flood regardless of orientation (that being said, I still believe vertical is the best orientation to vent any gas buildup as well as for priming). What would stop your liquid from finding equilibrium is pressure in the column above the volume of fluid. On my pump I have a tee on the outlet with a vertical ball valve for main pumping and a horizontal ball valve for priming/sampling during pumping.
When I flood my pump from my source volume, I open the priming/sampling valve to release air pressure and flood the pump. After the pump is flooded, it works fine.
Have you tried disconnecting your outlet hose from your destination and opening the outlet valve? The manufacturers generally advise against any angled fittings within a few inches of entering the pump, but I suspect you're having an issue with air pressure in your lines instead of restriction.
Angled fittings and hose ID certainly matter, but they should result in reduced performance not NO performance.