Unity – Checking if the player is grounded not working

Do not use OnTriggerStay to do this. That’s not guaranteed to be true very time.

Set isGrounded flag to true when OnCollisionEnter is called. Set it to false when OnCollisionExit is called.

bool isGrounded = true;

private float jumpForce = 2f;
private Rigidbody pRigidBody;

void Start()
{
    pRigidBody = GetComponent<Rigidbody>();
}

private void Update()
{
    if (Input.GetButtonDown("Jump") && isGrounded)
    {
        pRigidBody.AddForce(new Vector3(0, jumpForce, 0));
    }
}

void OnCollisionEnter(Collision collision)
{
    Debug.Log("Entered");
    if (collision.gameObject.CompareTag("Ground"))
    {
        isGrounded = true;
    }
}

void OnCollisionExit(Collision collision)
{
    Debug.Log("Exited");
    if (collision.gameObject.CompareTag("Ground"))
    {
        isGrounded = false;
    }
}

Before you say it doesn’t work, please check the following:

• You must have Rigidbody or Rigidbody2D attached to the player.

• If this Rigidbody2D, you must use OnCollisionEnter2D and
  OnCollisionExit2D.

• You must have Collider attached to the player with IsTrigger
  disabled.

• Make sure you are not moving the Rigidbody with the transform such
  as transform.position and transform.Translate. You must move
  Rigidbody with the MovePosition function.