Try rotating the crank about 90 degrees at a time then prime the pump. When I build my engines I use an engine pre-oiler which forces oil into the system under pressure. While that happens rotating the crank helps let oil get everywhere including up to the lifter bay and rockers.
Of course, I build real engines (Mopar) but I'm sure it will work on a Chevy.
it's caused by not having the body of the distributor in place while priming. The distributor body creates a link for the oil to pass from the vertical passage to the right hand oil galley. Some priming tools use an old base cut off to make that link.