Problem with jumping

Why CharacterController.isGrounded it's TRUE all time? And how to make more realistic jumping?

 using System;
 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 
 
 public class moveController : MonoBehaviour {
 
     public float speed = 17f;
     public float jumpHeight = 100f;
     public Animation anim;
     public Rigidbody rig;
     public CharacterController charcontroll;
     public bool isGround = true;
 
 
     public void Move(Transform transform)
     {
         var velocity = new Vector3();
         var movement = new Vector3();
 
         var inAirMultiplier = 0.25f;
 
         if ((Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.D)))
         {
             var z = Input.GetKey(KeyCode.W) ? 1.0f : 0;
             z = Input.GetKey(KeyCode.S) ? -1.0f : z;
 
             var x = Input.GetKey(KeyCode.D) ? 1.0f : 0;
             x = Input.GetKey(KeyCode.A) ? -1.0f : x;
 
             movement.z = speed * z;
             movement.x = speed * x;
 
             Vector3 keyboardPosition = new Vector3(x, 0, z);
 
             var targetRotation = Quaternion.LookRotation(transform.position + keyboardPosition - transform.position);
             transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.deltaTime * 2.0f);
 
             anim.CrossFade("run");
         }
 
         if(!((Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.D))) && !Input.GetKey(KeyCode.Space))
         {
             anim.CrossFade("idle");
         }
 
         velocity.y += Physics.gravity.y * Time.deltaTime;
         movement.x *= inAirMultiplier;
         movement.z *= inAirMultiplier;
 
         movement += velocity;
         movement += Physics.gravity;
         movement *= Time.deltaTime;
         transform.GetComponent<CharacterController>().Move(movement);
     }
 
 
     void FixedUpdate()
     {
         isGround = charcontroll.isGrounded;
         if (Input.GetButton("space") && isGround)
         {
             rig.AddForce(transform.up * jumpHeight);
             isGround = false;
         }
 
         Move(transform);
     }
 
     
 
 }