I have been tossing this around in my mind for a while now trying to figure out why your front skirts didn't reduce drag and I have a theory.
Let's see If I can explain.
Short version: They would probably show an improvement in combination with a front air dam.
Here is my attempt at the why. High pressure air flowing under a car is forced by the body overhang around the wheels and between them. The wheel wells are a path of least resistance escape to the lower pressure area on the sides of the car. If you reduce the amount of air coming into the crunch zone under the overhang to the area left between the wheels with an air dam, the rest of the air will go over the top and sides of the car.
If you just close off the wheel wells without changing anything else, you are just increasing the pressure gradient under the car and closing off an escape route, albeit a dirty one. I think this is the reason all the newer front ends you see have the higher middle section of the air dam. Block off the air that would have hit the wheels and leave enough to go between the wheels.
Look at the picture of the Opel Calibra in
this report.(Thx lunarhighway) The smoke shoots out from the middle to the sides toward the wheel at a 45˚ish angle and ends up going through the wheel wells and around the wheels into turbulence on the sides. It wants to go from the higher pressure to the lower but the wheels are in the way. Send it over and around rather than trying to ram it under the car.
That's my attempt at an explanation of my thoughts on front wheel skirts and air dams.