Collider doesn't work always

Hi there! I'm using this script for player (ball) movement and I'm using colliders to check if a player can jump or he cannot. But this colliders doesn't work always so as a result I have a situation when player IS on a ground but he CAN NOT jump. Any ideas? Thanks!

 using UnityEngine;
 using System.Collections;
 
 public class BallMovement : MonoBehaviour {
 
     public float speed;
     public float jumpSpeed;
     bool CanJump;
     bool jump;
    
     
 
     
     void Start () 
     {
         
         CanJump = true;
         jump = false;
        
     }
 
     void OnCollisionExit(Collision collisionInfo)
     {
         CanJump = false;
         jump = false;
 
       
     }
 
   
     void OnCollisionEnter(Collision collisionInfo)
     {
 
         CanJump = true;
 
     }
 
     void Update()
     { 
        if (Input.GetKeyDown(KeyCode.Space) && CanJump == true)
         {
            jump = true;
          
         }
     }
 
     
     void FixedUpdate () 
     {
        
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
 
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
 
         rigidbody.AddForce(movement * speed * Time.deltaTime);
 
         if (jump == true)
         {
             rigidbody.AddForce(Vector3.up * jumpSpeed);
         }
 
     }
  
 }