Inspect & clean the mould carefully ,inspect with magnification for burrs or misplaced parts that may hold the blocks open, mainly the guide pins on 6 bangers.
I use the Midway "drop out" spray and it works like a champ. After you get the burr problem taken care of, use some of that. Also I bolted a piece of pipe a little larger in ID than the tab on the Lee sprue plate. When ready to open the mold, just stick the tab into the pipe for excellent leverage to cut the sprues and then open the mold. No banging or smacking the plate necessary. Smithy.