I'd make sure that the gasket surfaces are all good and flat, esp make sure that the cover isn't warped. Also check that where the bolts go threw aren't crushed in either. The best luck that I've had with gaskets on the side of an engine is to use 3M yellow weatherstrip adhesive to glue the gasket to the cover. If you use this make sure that you have a even bead all the way around so the gasket is glued all the way around and doesn't make a space where oil could leak threw. Then make sure you clean the block good with the lacquer thinner or brake clean and clean the bolt holes good. I always also use a little bit of pipe sealer on the bolt threads. The only down side to using the 3M weatherstrip adhesive is its hard to get off, but now impossible. Usually a razor blade and a good wire wheel will take it off.

Good Luck
Don