Gravity not working with AI

Whenever I run this script, and the character goes over a hill or something, he descends really slowly. I'm not 100% sure what the moveDirection.normalized thing does, so that's probably the problem. Is there something I can add to this or change that will make the character it's attached to adhere to gravity?

 #pragma strict
 
 var theEnemy : Transform;
 var checkpoints : Transform[];
 var charController : CharacterController = theEnemy.GetComponent(CharacterController);
 var currentCheckpoint = 0;
 
 
 function Awake(){
 currentCheckpoint = 0;
 }
 
 function FixedUpdate(){
 
 
 var currentDistance = Vector3.Distance(checkpoints[currentCheckpoint].position, theEnemy.position);
 var speed = 10;
 
 var moveDirection = checkpoints[currentCheckpoint].position-theEnemy.position;
 
 if(currentCheckpoint < checkpoints.length){
 
 theEnemy.LookAt(checkpoints[currentCheckpoint]);
 if(currentDistance<10){
 currentCheckpoint++;
 }
 else{
 charController.Move(moveDirection.normalized * speed * Time.deltaTime);
 }
 
 }
 if(currentCheckpoint == checkpoints.length){
 currentCheckpoint = 0;
 }
 
 }