addChildAt for top layer confusion

if (score==10 && score<30 ){

var myMovieClipbg:winbg = new winbg();

myMovieClipbg.x = 0;

myMovieClipbg.y = 5;

addChildAt(myMovieClipbg,1);

}

//level 3

else if (score>30 && score<60){

lastmc.x = 0;

lastmc.y = 0;

addChildAt(lastmc, 1);

}

"lastmc" should be top layer background when score is greater than 30 . but its not working . moreover if i use addChild(lastmc) . it becomes top of all layer . so My hero and enemies everything go down. i can not play . but  need lastmc just top of myMovieClipbg background layer .

i assume the confusion coming from addChildAt

note addChildAt(myMovieClipbg,1); is working fine for second level. it brings the background top of first background .

but addChildAt(lastmc, 1); is failing to bring the background of of second background