If the pump works the ball has to come off the seat by pump pressure.
My one failure with check valves is on a C15 with the valve after the pump, it stuck shut and the pump would not force it open although it was primed. I caught it before it caused any damage as I was using clear oil line on a reassembled engine and was looking at the flow as a check. As the engine started oil flowed into the engine, then it slowed and next went into reverse followed by lots of air, only a new check ball and spring and seat refurb fixed it. Changing oil pumps had no effect and oil lines were all clear.
I know these are positive displacement pumps and the theory is they produce highest pressure when the exit is restricted but you can never beat seeing the result of a post pump blockage, instead of pumping oil out when it met resistance it must have got air in through a mating surface and that forced the oil back out the inlet.