I honestly think the problem is that fillies and mares sour very easily, it isn't because they want to be mommies necessarily, I'm more inclined to think they just are generally more sensitive and actually smarter than the colts. I see it even in show horses in all disciplines, you have to vary their routine, liketrail ride one day etc... I also think though on the other hand, when you have them figured out as to what makes them happy, that they will try their hearts out for you more than the colts... they are just very delicate, fragile minded creatures.
I remember Wayne Lukas saying the key to fillies was to treat them like your best girlfriend, they need to be pampered and spoiled, or else!