Why _visible=true doesnt work?

programming flash
1

I have an horizontal sliding mc.When the target position is reached, I set the buttons to _visible=false, they are no more useful because the end of slider is reached.But then I want to set them to _visible=true when the slider changes the position back.
Everything works except _visible=true.
Can someone tell me why?

import mx.transitions.Tween;
import mx.transitions.easing.*;
new Tween(mc, "_x", Bounce.easeOut, 1151, 0, 1, true);  
var target = 0;  
next.onRelease = function() {  
    target -= 770;  
    new Tween(mc, "_x", Strong.easeInOut, mc._x, target, 1, true);  
    if (target == -770) {  
        this._visible = false;
  trace(target)
    } else {  
        this._visible = true;  
    }  
};
prev.onRelease = function() {  
    target += 770;  
    new Tween(mc, "_x", Strong.easeInOut, mc._x, target, 1, true);  
    if (target == 0) {  
        this._visible = false;
                          trace(target)
    } else {  
        this._visible = true;  
    }  
};