simpleButtons dissapearing

here is my code

//game_display_bitmapData is an object which contains bitmap data. this bitmap data is loaded from an XML file.

var option_button:SimpleButton=create_a_button("grunge_button",50,360);
var about_button:SimpleButton=create_a_button("grunge_button",170,360);

function create_a_button(_string:String,_x:Number , _y:Number)
    var button:SimpleButton = new SimpleButton();
    button.upState=game_display_bitmapData[_string+"_up"]; //game_display_bitmapData is an object which contains bitmap data.
    button.x = _x
    button.y = _y
    return button;


Now what happens is tht only one button is visible or workin at any point of time. But if i use say, “xyz_button” as the params for the about_button. then it works fine. i can see both the buttons.
also i tried giving unique names to the button, even that didnt work. why is this happening?