Player no longer jumping

Hi,

I'm working on my first project and I set up a Third Person Movement script which was working very well, but I seemed to have broken something.

I can no longer Jump even though the debug log shows that I'm grounded and jump is pressed down. The character controller does show a velocity change in the Y direction, but the character is stuck to the ground. The Y position moves in the up position by a barely noticeable amount, as if the character is stuck.

I can only get it to work if I set isGrounded to NOTisGrounded:

If I set "if (Input.GetButtonDown("Jump") && !isGrounded)", it works. However once the groundDistance is reached the velocity changes to my default -2.

Hope someone can help, this is driving me nuts! Cheers

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ThirdPersonMovement : MonoBehaviour
 {
     public CharacterController controller;
     public Transform cam;
 
     public float speed = 2f;
     public float turnSmoothTime = 0.1f;
     public float gravity = -9.81f;
     public float jumpHeight = 3f;
 
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;
     float turnSmoothVelocity;
 
     Vector3 velocity;
     bool isGrounded;
 
     // Update is called once per frame
     void Update()
     {
         isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
 
         if (isGrounded)
         {
            Debug.Log("Is Grounded");
         }
 
         if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
         }
 
         float horizontal = Input.GetAxisRaw("Horizontal");
         float vertical = Input.GetAxisRaw("Vertical");
         Vector3 direction = new Vector3(horizontal, 0f, vertical).normalized;
 
         if (direction.magnitude >= 0.1f)
         {
             float targetAngle = Mathf.Atan2(direction.x, direction.z) * Mathf.Rad2Deg + cam.eulerAngles.y;
             float angle = Mathf.SmoothDampAngle(transform.eulerAngles.y, targetAngle, ref turnSmoothVelocity, turnSmoothTime);
             transform.rotation = Quaternion.Euler(0f, angle, 0f);
 
             Vector3 moveDir = Quaternion.Euler(0f, targetAngle, 0f) * Vector3.forward;
             controller.Move(moveDir.normalized * speed * Time.deltaTime);
         }
 
         if (Input.GetButtonDown("Jump") && isGrounded)
         {
             Debug.Log("Is Jumping");
             velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
         }
 
         velocity.y += gravity * Time.deltaTime;
 
         controller.Move(velocity * Time.deltaTime);
 
     }
 }