Consistency is a terrible thing. I generally use humbrol undercoats and then fine oils on top. I have found that humidity, drying times and solvent content are the biggest variables. Not all paints and paint colours dry at the same rates. The space of 1/2 hour has a big effect at times. Personally, I like different colour reactions as it gives each figure a unique look.
The most unusual problem lately is that my Humbrol matt 63 is sometimes drying matte, and sometimes drying glossy. I think it is reacting to the undercoat, but still trying to figure it out. (Get it, FIGURE it out)