Player not responding to Jump Input

(I'm really new at Unity! Please be forgiving!) I'm working on a tutorial series where I have the Character Controller scripted to do some basic movement with the arrow keys and WASD with the spacebar set to make the player jump. Everything works except the jump. Not really sure how to fix this. Any ideas? Here's my code:

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovement : MonoBehaviour 
 {
     CharacterController _controller;
     
     [SerializeField]
     float _moveSpeed = 5.0f;
     
     [SerializeField]
     float _jumpSpeed = 20.0f;
     
     [SerializeField]
     float _gravity = 1.0f;
     
     float _yVelocity = 0.0f;
 
     // Use this for initialization
     void Start () 
     {
         _controller = GetComponent<CharacterController>();
     }
     
     // Update is called once per frame
     void Update () 
     {
         Vector3 direction = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
         Vector3 velocity = direction * _moveSpeed;
         
         if (_controller.isGrounded)
         {
             if (Input.GetButtonDown("jump"))
             {
                 _yVelocity = _jumpSpeed;
             }
         }
         else
             {
                 _yVelocity -= _gravity;
             }
         
         velocity.y = _yVelocity;
         
         _controller.Move(direction * Time.deltaTime);
     }
 }