Button loads MC but does not play it?

programming flash
1

Ok, so i have been learning little by little some AS through Kirupa, but i still need the help. So here is my issue i have this code below that allows me to switch between images using a mc button, but when i press, the mc loads but does not play the fade in? Can someone explain what is missing in this code that skip the play?

stop();
MC1._visible = true;
MC2._visible = false;
Btn2.onPress = function(){ //brings up the second movieclip
MC1._visible = false;
MC2._visible = true;
}
Btn1.onPress = function(){ //brings up the first movieclip
MC2._visible = false;
MC1._visible = true;
}