No oil flow mystery

Remove the oil pressure sender, and crank the engine (filter on). See if you get any oil flow. If no, problem is in the pump/filter area. If yes (might be reduced flow) problem is internal. The main oil gally has a threaded plug in each end. If the rear plug is left out, oil will run out the bell housing area. If the front plug is left out, the oil will stay inside the timing cover and run back into the pan.
EDIT: Did you try a different filter?