Whats wrong with this?

1

this has been driving me nuts for the past hour and i just can’t find the problem with it.

import mx.transitions.*;
import mx.transitions.easing.*;


earth._alpha = 0;
wind._alpha = 0;
fire._alpha = 0;


btn_earth.onRollOver = function():Void {
	earth._alpha = 100;
	var twMoveMc1:Tween = new Tween(earth, "_height", Elastic.easeOut, 0, 120, 1, true);

}

btn_earth.onRollOut = function():Void {
	var twMoveMc2:Tween = new Tween(earth, "_height", Strong.easeOut, 120, 0, 1, true);

}




btn_wind.onRollOver = function():Void {
	wind._alpha = 100;
	var twMoveMc3:Tween = new Tween(wind, "_height", Elastic.easeOut, 0, 120, 1, true);
}

btn_wind.onRollOut = function():Void {
	var twMoveMc4:Tween = new Tween(wind, "_height", Strong.easeOut, 120, 0, .8, true);

}




btn_fire.onRollOver = function():Void {
	fire._alpha = 100;
	var twMoveMc5:Tween = new Tween(fire, "_height", Elastic.easeOut, 0, 120, 1, true);
}

btn_fire.onRollOut = function():Void {
	var twMoveMc6:Tween = new Tween(fire, "_height", Strong.easeOut, 120, 0, .8, true);

}

when i test the movie, after a while the square stops animating smoothly and some just stay on stage.

PLEASE HELP.