Input Touch Not Working Quite Right...

Ok, I'm trying to make it so that anytime the user has to hold the bottom of the screen to "float up" & then they would tap the top, while holding the bottom to get them to "float down". Ideally I'd like it to work vice versa too & if they could hold the top to "float down" & then tap the bottom to "float up". Right now, it works sometimes, but not always & I can't even tell you what triggers it to work. Sometimes you can touch the bottom & you'll be floating up but tapping up top doesn't do anything.

Here's my code. Any suggestions?

 #pragma strict
 
 var jumpSpeed : float = .002;
 var fallSpeed : float = -.004;
 var position : Vector2;
 
 function Start() {
 
 }
 
 function Update() {
 
 
 //  if (Input.GetKeyDown("space")) {
     if (Input.touchCount>0) {
         var touch : Touch = Input.GetTouch(0);
         if(touch.position.y < Screen.height/2) {
             GetComponent.<Rigidbody2D>().velocity.y = jumpSpeed;
                 } else {
                     GetComponent.<Rigidbody2D>().velocity.y = fallSpeed;
                 }
             }
         }