One thing I have seen that will cause this is lifter bore clearance. On my race engine I have a little more lifter bore clearance than I like and I have heck trying to get oil to the rocker arms with a primer. Zero problems once the engine is running. This can be easily verified with the intake off, but little tougher with it on. I can't say with absolute certainty that this is your problem, just something else to look at. I was surprised at how much oil the lifter bores will dump when you are spinning it with a primer.
Curtis