Many times a valve cover will leak because the cover flange along one edge is slightly deformed (bent or warped). You can check it by laying a straight edge along the flange when you have the valve cover off. Also check the head surface where the valve cover gasket lies...sometimes this surface is not machined too well at the factory (common on many brands of car, but more common on cars with iron heads). You can usually clean up any small imperfections on the head's valve cover gasket surface with a fine-cut metal file, or even sandpaper.
If the valve cover flange is warped too badly it might not hold the gasket flush against the head surface, and thus keep it from leaking some oil. In a case like this, gasket sealer may not fix the problem...the only remedy may be a new valve cover.
It is best to use a small torque wrench to tighten valve cover bolts then you replace the gasket, so that you minimize the risk of deforming the valve cover flange.