If you do a fresh engine build, or even the pump. The pump and oil system will want priming. Without having the engine running you can remove the oprv and kick the engine over (with the plugs out) until you see oil coming out of the oprv port. Once you have done this you have confirmed that the oil lines the correct way around and that within a short space of time the crank will be full of oil and feeding the big ends.

Did you assemble the motor with engine oil or engine assembly lube on the big end shells? Engine assembly oil is like treacle but stays with the journal until sufficient oil flow washes it away.