worst case scenario I would start from the bottom and trace all your oil passages up checking for anything out of the ordinary- if nothing works I would try back flushing the oil passages to see if anything goes through the other way.
Other obvious questions that I might have had happen to me in the past or can think of off the top of my head-
Oil reads proper level or a little high on the dipstick-
Pre filled the filter-
if so equipped check valves are clean and working-
All fittings tight on pump and pump installed correctly-
Check to see with pan off is your special tool correctly contacts oil pump drive?
Just things I have thought of.
