Why is UpdateJump not causing my character to jump - unity3d

Question

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class PlayerController : MonoBehaviour
{
[SerializeField] Transform playerCamera = null;
[SerializeField] float xSensitivity = 4.0f;
[SerializeField] float ySensitivity = 4.0f;
[SerializeField] float moveSpeed = 6.0f;
[SerializeField] float gravity = -13.0f;
[SerializeField][Range(0.0f, 0.1f)] float moveSmoothTime = 0.05f;
[SerializeField][Range(0.0f, 0.2f)] float mouseSmoothTime = 0.03f;
[SerializeField] bool lockCursor = true;
float cameraPitch = 0.0f;
float velocityY = 0.0f;
CharacterController controller = null;
Vector2 currentDir = Vector2.zero;
Vector2 currentDirVelocity = Vector2.zero;
Vector2 currentMouseDelta = Vector2.zero;
Vector2 currentMouseDeltaVelocity = Vector2.zero;
public Vector3 jump;
public float jumpForce = 8.0f;
public bool isGrounded;
Rigidbody rb;
void Start()
{
rb = GetComponent<Rigidbody>();
jump = new Vector3(0.0f, 2.0f, 0.0f);
controller = GetComponent<CharacterController>();
if(lockCursor)
{
Cursor.lockState = CursorLockMode.Locked;
Cursor.visible = false;
}
}
// Update is called once per frame
void Update()
{
UpdateMouseLook();
UpdateMovement();
OnCollisionStay();
UpdateJump();
}
void UpdateMouseLook()
{
Vector2 targetMouseDelta = new Vector2(Input.GetAxis("Mouse X"), Input.GetAxis("Mouse Y"));
currentMouseDelta = Vector2.SmoothDamp(currentMouseDelta, targetMouseDelta, ref currentMouseDeltaVelocity, mouseSmoothTime);
cameraPitch -= currentMouseDelta.y * ySensitivity;
cameraPitch = Mathf.Clamp(cameraPitch, -70.0f, 70.0f);
playerCamera.localEulerAngles = Vector3.right * cameraPitch;
transform.Rotate(Vector3.up * currentMouseDelta.x * xSensitivity);
}
void UpdateMovement()
{
Vector2 targetDir = new Vector2(Input.GetAxisRaw("Horizontal"), Input.GetAxisRaw("Vertical"));
targetDir.Normalize();
currentDir = Vector2.SmoothDamp(currentDir, targetDir, ref currentDirVelocity, moveSmoothTime);
if (controller.isGrounded)
velocityY = 0.0f;
velocityY += gravity * Time.deltaTime;
Vector3 velocity = (transform.forward * currentDir.y + transform.right * currentDir.x) * moveSpeed + Vector3.up * velocityY;
controller.Move(velocity * Time.deltaTime);
}
void OnCollisionStay()
{
isGrounded = true;
}
void UpdateJump()
{
if (Input.GetKeyDown(KeyCode.Space) && isGrounded)
{
rb.AddForce(jump * jumpForce, ForceMode.Impulse);
isGrounded = false;
Debug.Log("JUMP");
}
}
}
In my script I can't seem to get the UpdateJump function to work as intended. When I press space I get the console message but the rb.AddForce appears to do nothing. what is the reason that I can't get my player controller to jump and how can I go about fixing this and making sure it doesn't occur again further down the line.