Unity 5: UI button OnPointerDown not function as expected

I just started learning Unity for few days. I am testing the function of UI Arrow Button which is supposed to be able to translate a car object in x and z position in a 3D scene.

Since my target platform is WebGL, I also coded the keyboard control of arrow button. Currently I wrote 2 scripts for this function. One is attached to the car object and another is attached to the UI Up Arrow Button.

The keyboard control worked fine in PC but I would like to enable the control of translation of the car object also in mobile (though Unity WebGL doesn't officially support mobile platforms). I attempted to use the OnPointerDown() function to get it work but it wasn't even called when I pressed the up button.

How to get the UI arrow button work when it is pressed down?

The Image of the Interface is here.

And my scripts are here:

the one attached to car object

 using UnityEngine;

 public class CarPlayerControl : MonoBehaviour {

 public float speed = 40;
 private bool touchCtrl = false;
 private float xInput = 0f;
 private float zInput = 0f;

 void Awake(){
     //if (Input.touchSupported && Application.platform 
          //     != RuntimePlatform.WebGLPlayer){
         //touchCtrl = true;
         //}
         }
 
 void FixedUpdate(){

     xInput = 0f;
     zInput = 0f;

     if (touchCtrl)
     {
         //if (Input.GetButton("ButtonUp"))
         //{
         //    xInput = 1.0f;
         //    Debug.Log("button up " + xInput);
         //}
         //else if (Input.GetButton("ButtonDown"))
         //{
         //    xInput = -1.0f;
         //    Debug.Log("button down " + xInput);
         //}
         //else if (Input.GetButton("ButtonLeft"))
         //{
         //    zInput = 1.0f;
         //    Debug.Log("button left " + zInput);
         //}
         //else if (Input.GetButton("ButtonRight"))
         //{
         //    zInput = -1.0f;
         //    Debug.Log("button right " + zInput);
         //}
     }
     else //mouse
     {
         xInput = Input.GetAxis("Horizontal");
         zInput = Input.GetAxis("Vertical");
     }

     Move(xInput, zInput);
 }

 public void Move(float xInput, float zInput)
 {
     float xMove = xInput * speed * Time.deltaTime;
     float zMove = zInput * speed * Time.deltaTime;

     float x = KeepXWithinRange(xMove);
     float y = transform.position.y;
     float z = KeepZWithinRange(zMove);

     transform.position = new Vector3(x, y, z);
 }

 private float KeepXWithinRange(float xMove){
     float x = transform.position.x + xMove;
     return Mathf.Clamp(x, 0, 900);
 }

 private float KeepZWithinRange(float zMove)
 {
     float z = transform.position.z + zMove;
     return Mathf.Clamp(z, -470, 500);
 }
 }

and the one attached to the UI Up arrow button

 using UnityEngine;
 
 public class ButtonUpControl : MonoBehaviour, IPointerDownHandler, IPointerUpHandler
 {
 public GameObject car;
 private CarPlayerControl carPlayerControl;
 private bool mouseDown;
 private float xInput;

 void Awake()
 {
     carPlayerControl = car.GetComponent<CarPlayerControl>();
     mouseDown = false;
 }
 void Start()
 {

 }

 void FixedUpdate()
 {
     if (mouseDown) {
         xInput = 1.0f;
     }
     else {
         xInput = 0f; 
     }

     carPlayerControl.Move(xInput, 0f);
 }

 public void OnPointerDown(PointerEventData eventData)
 {
     mouseDown = true;
     Debug.Log("on ptr down btn up ");
 }

 public void OnPointerUp(PointerEventData eventData)
 {
     mouseDown = false;
     Debug.Log("on ptr up btn up ");
 }
 }