Basic Health Bar Display Problem

I have a simple health bar script attached to an enemy. When I press a key the enemy health and health bar are supposed to reduce in size. To a certain extent this happens but not exactly as I would like.

When I press the key the enemy's health decreases as expected. But the size of the bar only decreases when the enemies health gets below half!

I have some heavily commeneted code that does this.

First the enemyHealth script

var currHealth = 100; //currentHealth var maxHealth = 100; //maxHealth var healthBarLength; //length of healthbar

function Start() { healthBarLength = ((Screen.width/2)/(maxHealth / currHealth));//set default length }

function Update () {

}

function OnGUI() { GUI.Box(Rect(10,40,healthBarLength,20), currHealth + "/" + maxHealth);//draw health bar

}

//method for adjusting the health bar function AdjustHealthBar() {
if ((currHealth > 0) && (currHealth <= maxHealth)){ healthBarLength = ((Screen.width/2)/(maxHealth / currHealth)); } else { healthBarLength = 0; } }

//accessor for adjusting health function AdjustCurrentHealth(adj) { currHealth += adj; AdjustHealthBar(); }

Second the small piece of code which reduces the health (attacks):

var target : GameObject; //the player's target

function Update () { if (Input.GetKeyUp(KeyCode.F)) { Attack(); } }

function Attack() { var eh = target.GetComponent("EnemyHealth"); //var for enemy health script

eh.AdjustCurrentHealth(-10); //take 10 off enemy health }

I cannot figure out why this does not work and why I get the seemingly random result of the health bar staying the same length until health gets to 50/100.