Assuming it was fresh stock and not sitting on someone's shelf forever before you installed it, I can only think of a couple things that might cause what you're experiencing:
1) overtightening
2) the flange on the cover might be bent up from having been previously overtightened
3) are your draft tube and oil filler cap clean and allowing the engine to breathe properly?
4) are the oil drain holes clear in the cylinder head?
Lou Manglass