Hi Chef, I usually remove the delivery valve and holder. Check that the D valve is free to move in the body.
Next, push a small brass drift against the plunger (located in the center of the area beneath the delivery valve.)
Turn the engine a few revolutions, you should feel the plunger move back and forth. If no movement is felt, the plunger is stuck. Liquid Wrench is my Penetrating oil of choice.
If the plunger does not free itself, a gentle rap on the brass drift might be required to free it. You may have to turn the engine in a few different positions to ensure that the cam lobe is not directly under the pump.