I've had something similar happen & it was the gasket between the oil pickup & the block. It had fallen off one of the bolts when I was doing it up & was blocking the gallery up into the block. Maybe the same thing has happened to you. The bad news is that I had to take the motor out to check & repair. The cause of my mistake was I had a baffled sump & I had to put the pickup through it before fitting either to the block.Maybe you could remove the oil pump & drop the sump, then get a mirror on a stick likes dentists use to inspect the pick up gasket. I suggest this because your obviously not getting oil from the sump.