Issue when terrain or ground is not plane, a bug or programming?

I have a sphere object and it's jumping is really weird. I really don't know if it has to do with my code or just a bug. When my sphere jump on a plane ground my jumping works nice, but if i edit and put a hill on the terrain i got a short jump. As long as the mesh is not plane it will short jump and I don't really know why. This sphere might be retarded. Here's my code.

public class BasicMovement : MonoBehaviour {

 public float speed = 30;
 public float speedSlowdown = 5;
 public float desiredMaxSpeed = 5;
 public float jumpHeight = 8.0f;
 
 private Rigidbody rb;
 private bool grounded;
 private bool isjumping;
 private bool isMoving;
 
 void Start()
 {
     rb = GetComponent<Rigidbody>();
 }
 
 void OnCollisionStay(Collision col)
 {
     grounded = true;
     isMoving = true;
 
 }
 
 void OnCollisionExit(Collision col)
 {
     grounded = false;
     isMoving = false;
 
 }
 
 void Update()
 {
     if (Input.GetKeyDown(KeyCode.Space) && grounded == true)
     {
         isjumping = true;
     }
 }
 
 void FixedUpdate()
 {
     if (isMoving == true)
     {
        Movement(speed);
        limitSpeed(desiredMaxSpeed);
     }
     else
     {
         Movement(speedSlowdown );
     }
     if (isjumping)
     {
         BallJumping();
     }
 }
 
 void limitSpeed(float maxSpeed)
 {
     if (rb.velocity.magnitude > maxSpeed)
     {
         rb.velocity = rb.velocity.normalized * maxSpeed;    
     }
 }
 
 void  Movement(float currentSpeed)
 {
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");
    Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
    rb.AddForce(movement * speed);
 
 }
 void BallJumping()
 {
     rb.AddForce(Vector3.up*jumpHeight,ForceMode.VelocityChange );
     isjumping = false;
 }

}