If simple green won't do the job, try something that's more heavy duty (such as Engine brite, or an engine cleaning product called Steam, which kicks ass BTW)
if you are really having trouble try this:
Easy Off oven cleaner spray. spray it on the valve cover, try to keep it off the plastics, as they will get marks on them if it dries on the plastic and rubber bits.
run the motor, let it get to normal operating temperature. when it's there, let it idle for a minut or two. shut off the car, let the motor cool a little bit (about a minute or so will do) and hose it off. be sure to cover any Intakes
, eletrical bits, ignition wires, etc, with plastic wrap too. this goes for all engine cleaning products.
Last ditch solution, if all else fails, buy some aluminum polish (you know, for rims and such, at an auto parts store) and polish the valve cover with it. that'll get it off for sure with some elbow grease.