I had the same issue last week with a new round oil tank, oil cooler, and -AN lines. The engine was dry and I had rebuilt the oil pump. Of course I forgot to prime it before putting it in the engine. To top it off, I put a cyclexchange charging system on, so no more starter.
I figured I'd put some oil in the engine through the tappet covers just to get some lube in there. After cranking it over witht he kickstarter for a few minutes, I had nothing flowing through. Oil was making it fromt he oil tank down the hose and to the pump, but the pump was airlocked.
I got oil flow/pressure by blowing into the breather tube while turning the engine with the kickstarter. Within 10 kicks, pressure started to build and the oil level in the tank dropped as it filled the oil filter and oil passages.