Moving a cube with RigidBody. MovePostition, it randomly stops moving

When I press the W key, the cube starts going forward, but sometimes it suddenly stops. When I press the W or S key, it doesn't move. It only starts moving when I rotate the cube, but it can stuck again. What causing this issue?

I control the cube in the Ember class. using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class Emberscript : MonoBehaviour {
 
     public class Ember
     {
         public Transform body;
         //public Transform lArm, rArm;
         public Vector3 movingVector = new Vector3(0, 0, 0);
         public int rotation = 0;
         public bool falling = false;
         public Rigidbody rigidbody;
 
         public Ember(Transform body, Transform lArm, Transform rArm)
         {
             this.body = body;
             //this.lArm = lArm;
             //this.rArm = rArm;
         }
 
         public void Move()
         {
             rigidbody.MovePosition(rigidbody.position + Quaternion.Euler(0, rotation, 0) * movingVector * Time.fixedDeltaTime);
 
             body.rotation = Quaternion.Euler(0, rotation, 0);
         }
 
         public void Jump()
         {
             rigidbody.AddForce(Vector3.up * Mathf.Sqrt(2 * -2f * Physics.gravity.y), ForceMode.VelocityChange);
         }
     }
 
     void UserInput()
     {
         if (Input.GetKeyDown(KeyCode.W))
         {
             ember.movingVector += new Vector3(0, 0, 5f);
         }
         if (Input.GetKeyUp(KeyCode.W))
         {
             ember.movingVector -= new Vector3(0, 0, 5f);
         }
         if (Input.GetKeyDown(KeyCode.S))
         {
             ember.movingVector += new Vector3(0, 0, -5f);
         }
         if (Input.GetKeyUp(KeyCode.S))
         {
             ember.movingVector -= new Vector3(0, 0, -5f);
         }
         if (Input.GetKey(KeyCode.A))
         {
             ember.rotation += -1;
         }
         if (Input.GetKey(KeyCode.D))
         {
             ember.rotation += 1;
         }
         if (Input.GetKeyDown(KeyCode.Space))
         {
             ember.Jump();
         }
     }
 
     void SpawnCircle(float r)
     {
         float cubeLength = 2 * r * Mathf.PI / 360;
         for (int i = 0; i < 360; i++)
         {
             GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
             cube.transform.position = Quaternion.Euler(0, i, 0) * new Vector3(0, 0, r);
             cube.transform.localScale = new Vector3(cubeLength + 0.05f, 1f, 1f);
             cube.transform.rotation = Quaternion.Euler(0, i, 0);                
         }
     }
 
     Ember ember;
     public Transform lArm, rArm;
     int numberOfCircles = 0;
 
     void Start () {
         ember = new Ember(transform,lArm,rArm);
         ember.rigidbody = ember.body.GetComponent<Rigidbody>();
     }
     
     void Update () {
         UserInput();
 
         if(Vector3.Distance(ember.body.position, Vector3.zero) - numberOfCircles * 5 > 0)
         {
             numberOfCircles++;
             SpawnCircle(numberOfCircles * 5);
         }
 
         Debug.Log(ember.rigidbody.position + Quaternion.Euler(0, ember.rotation, 0) * ember.movingVector);
     }
 
     void FixedUpdate()
     {
         ember.Move();
     }
 }