If I remember correctly, the oil comes in through a hole in the side of bore, not the back, so the hole in the middle of the plug has no effect. When you drive the plug in, the side of it covers the hole where the oil comes out, stopping the flow at that point. Feel around in the bore where the plug goes and you should be able to feel the small hole where the oil comes out.

This should definitely help with the oil pressure problem. Let me know if you need more help.