My own preference for pre-oiling is to to use a speed handle instead of an electric drill. I’ve picked out problems with oil pumps that would not have been noticed with an electric drill but can be felt when hand turning a speed handle. No more than thirty-forty seconds by hand cranking and beginning with an empty filter is the norm in getting oil pressure built up and oil flow visible at the rocker arms if a grooved camshaft is being used. Like Tim mentions, if the cam is crossdrilled, then the engine may have to be rotated to get oil to the topend. Regardless of the method used to turn the oil pump, once oil pressure builds up, the drive to the oil pump will load enough that you can feel it. Using an electric drill will obviously get oil pressure much quicker than my 30-40 seconds.
As a rule, the oil pump needs lube in it before pre-oiling the engine can take place. If the pump is assembled dry and installed on the engine like this then the rotors do not seal adequately at first to get that initial pull of oil started. Removing the pump and shooting a few squirts of oil into the inlet side of the pump will usually fix this. Tim's suggestion of filling the chamber behind the oil filter plate with oil is also a good one.
Another issue to consider is if the rubber grommet on the inlet tube is not seated correctly which causes air to be pulled in at that point and in turn prevents any oil flow. If the oil inlet tube is bent and not square with the entrance into the pump, then this can also present an opportunity for an air leak around the rubber grommet.

Lorena, Texas (South of Waco)