problem on Z axis ,cube not go forward....need help.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class plaeeeeer : MonoBehaviour {
[Range(0.2f, 2f)]
public float moveSpeedModifier = 0.7f;
float dirX, dirZ;
public Rigidbody rb;

// Use this for initialization
void Start () {
	
}

// Update is called once per frame
void Update () {
    dirX = Input.acceleration. x * moveSpeedModifier;
    dirZ = Input.acceleration.z * moveSpeedModifier;
}
private void FixedUpdate()
{
    rb.velocity = new Vector3(rb.velocity.x + dirX, rb.velocity.z + dirZ);
}

}

cube is still going