Was it mocked up without the crank, but with the bearings to make sure the thrust surface(s) line up squarely (comparing upper to lower)?
Was the main cap just torqued down, or was the thrust bearing seated first? (referring to forcing the crank forward in the block during torque down)
Either of these can cause an oiling issue. If the bearing shell thrust surfaces are not square with each other, the oil film gets scraped off the thrust surface....
