AI script not using gravity

When my zombie spawns(a bit off the ground) and i am farther than the max distance than the zombie does not fall or use gravity. Does any one know how to fix this.

here is my code:

using UnityEngine; using System.Collections;

public class Zombie : MonoBehaviour {

 public Transform Player;
 public GameObject _zombie;
 
 public float Movespeed = 2;
 
 public float MaxDistance = 30;
 
 private float CurrentHealth;
 private float MaxHealth = 100;
 
 void Start () {
 CurrentHealth = MaxHealth;
 Player = GameObject.FindGameObjectWithTag("Player").transform;
 _zombie = gameObject;
 }
 
 void Update () {
 
     
 if(CurrentHealth < 1){
 
 if(Network.isServer){
     Network.Destroy(gameObject);
 }
 else if(Network.isClient){
     Network.Destroy(gameObject);        
 }
 else{
     Destroy(gameObject);    
 }
 }
 
 float dist = Vector3.Distance(Player.position, _zombie.transform.position);
 PlayerController PC = Player.GetComponent();
 Player PS = Player.GetComponent();
 
 if(PS.isReloading){
     MaxDistance += 2;    
 }
 if(PC.walkingstate == WalkingState.Sneaking){
     MaxDistance = 15;        
 }
 else if(PC.CharCont.velocity.magnitude < 0.1f){
     MaxDistance = 20;    
 }
 else{
     MaxDistance = 30;        
 }
 if(dist < MaxDistance){
 transform.LookAt(new Vector3(Player.transform.position.x, transform.position.y,Player.position.z));
 CharacterController controller = GetComponent();
 controller.SimpleMove(_zombie.transform.forward * Movespeed);
 _zombie.animation.Play("ZombieWalk1");
 }
 
 }
 
 void ApplyBulletDamage( float WeaponDamage){
     CurrentHealth -= WeaponDamage;
 }

}