Florida,
As Shipwright mentioned, make sure the valve cover isn't bent. It is real common for the area around the bolt/screw holes to get mashed from overtightening. The standard procedure is to lay the valve cover on the sharp edge of a counter or something solid, and with cover upside down, lightly hammer the flange and screw hole areas FLAT! Spray gasket and valve cover with a little Hi-tack, assemble and you are off to the races.
Stuart.
