'89 roller-cam LA 360 w/Magnum heads - new roller lifters not pumping up

Is there an oil galley plug in the back that may be missing?

Already checked for it. Present and accounted for.

Honestly, I doubt a restriction in the galleries. It doesn't explain why I can move any lifter to any position in the engine and replicate the same results from each specific lifter, regardless of position (e.g., no flow, dribble, partial flow, or "Holy $&!^ that's the flow I'm looking for" flow).

-Kurt