This is what I would do, take the pan off and make sure it not warp and the bolt hole are not indented into the inside. Make sure there isn't any remaining gasket material on either surface. Then I would find a good contact gasket glue and glue the gasket to the the pan cover. Next I would grease the gasket on the tranny side and install and tighten to the recommended torque specs. If that doesn't work I would start looking for hairline cracks in the pan or your case. Good Luck