why cant enamel paint over enamel?

9 times out of 10 this usually happens on the panel the blend is done on.Back in the acrylic enamel days there was a blender used to "melt" the old to the new. In my experience this was the area that had faded or checked!! I have sprayed enamel over enamel many times and never had it check or fade but the surface that you are spraying on must be fully cured.
Just my 2 cents!!!