Can't jump when moving backwards and to the right or left (C#)

I've been working on this 3d shooter project for quite a while now, but a major problem that has always stumped me was the issue that I could not jump when moving backwards and right (or backwards and left). I can jump just fine any other direction, just not those specific directions. Any help is appreciated: Thanks!

Here's the code:

 using UnityEngine;
 using System.Collections;
  
  public class FPSControllerNew : MonoBehaviour {
      //Variables
      public float speed = 5.0F;
      public float jumpSpeed = 12.0F; 
      public float gravity = 20.0F;
      public float sensitivity = 2f;
 
      public bool isCrouching = false;
 
      private Vector3 moveDirection = Vector3.zero;
 
      public GameObject camera;
      public GameObject upperbody;
      public GameObject WeaponInHand;
 
 
      public Collider playerCollider;
      public Renderer playerRenderer;
 
 
      float rotX;
      float rotY;
    
 
 
      Transform cameraTransform;
 
    
 
      void Start ()
      {
         
         cameraTransform = Camera.main.transform;
      }
      
  
      void FixedUpdate() {
          CharacterController player = GetComponent<CharacterController>();
 
 
          player.transform.Rotate (0, rotX, 0);
          // is the player on the ground?
          if (player.isGrounded) {
              //Feed moveDirection with input.
              moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
              moveDirection = transform.TransformDirection(moveDirection);
              
              //Multiply it by speed.
              moveDirection *= speed;
             
 
              
          }
 
           //Jumping
              if (Input.GetKey(KeyCode.Space) && player.isGrounded)
              {
                  Debug.Log("Jumping");
                  moveDirection.y = jumpSpeed;
              }
 
 
          //Applying gravity to the player
          moveDirection.y -= gravity * Time.deltaTime;
          //Making the character move
          player.Move(moveDirection * Time.deltaTime);