how to move a platform up and down

i wrote a code to move a platform up and down but no object seems to stay on it i.e objects go through it? pls help...because it seems that the script is disabling the platform...

private var Xpos : float; private var Ypos : float; private var max : boolean;

var Vert : boolean; var maxAmount : int; var step : float;

function Start () { Xpos = transform.position.x; Ypos = transform.position.y; //collider.enabled = false; }

function Update () {

if(Vert) { if(transform.position.y >= Ypos + maxAmount){ max = true;} else if(transform.position.y <= Ypos) { max = false;}
} else{ if(transform.position.x >= Xpos + maxAmount){ max = true;} else if(transform.position.x <= Xpos) { max = false;}
}

 if(Vert){
   if(!max) {
      transform.position.y += step;}
    else{
      transform.position.y -= step;}
    }
 else{
    if(!max){  
      transform.position.x += step;}
    else{
      transform.position.x -= step;}  
 }
 }