Rigidbody 2D loosing velocity

Hello! I'm working on my mobile game and I have some trouble with moving the player. The game mechanics are simple: The player is running upwards and your target is to click the screen to jump between walls to avoid obstacles. I have a problem with moving the player. Here's the player script:

 public class Player : MonoBehaviour 
 {
 
     public float speed = 3.0f;
     public float maxSpeed = 10f;
     public Sprite[] sprite;
     public Sprite[] skins;
     public int[] alias;
     private Animator animator;
     private Rigidbody2D rb;
     private bool canJump = true;
     public bool canMove = true;
     public bool scorePoints = true;
     public int direction;
     private float cooldown;
     private float immortal = 0;
     private Game game;
     private Rect touchArea;
     private GameObject shield;
     private bool shieldIsActive;
     public GameObject shieldBtn;    
     private bool cantUseShield;
     private AudioSource jumpSound;
 
     void Start ()
     {    
         
         int skinId = PlayerPrefs.GetInt ("SelectedSkin", 0);
         Debug.Log (skinId);
         touchArea = new Rect(0, Screen.height * 0.15f, Screen.width, Screen.height * 0.70f);
     
         if(PlayerPrefs.HasKey("SoundMuted"))
             GetComponent<AudioSource>().mute = true;
         else
             GetComponent<AudioSource>().mute = false;
 
         jumpSound = GetComponent<AudioSource>();
 
         sprite [0] = skins [alias[skinId]];
         sprite [1] = skins [alias[skinId] + 1];
 
         game = Camera.main.GetComponent<Game>();
         animator = GetComponent<Animator> ();
         rb = GetComponent<Rigidbody2D> ();
         shieldBtn = GameObject.FindGameObjectWithTag("ShieldBtn");
         animator.speed = 1.5f;
 
         if (Mathf.FloorToInt (Random.Range (-1, 1)) == 0)
             direction = 1;
         else
             direction = -1;
         
         InvokeRepeating("Acc", 0.25f, 0.25f);
         Jump (10);
         Flip ();
     }
 
     void Update ()
     {    
         Debug.Log(rb.velocity);
         if (Input.anyKeyDown || Input.touchCount > 0) {
             Touch touch = Input.touches [0];
             if (touch.phase == TouchPhase.Began && touchArea.Contains(touch.position)) {
                 if (canJump == true && Time.time > cooldown && game.pause == false) {
                     jumpSound.Play();
 
                     cooldown = Time.time + 0.05f;
                     Jump (20);
                     Flip ();
                 }
             }
         }
 
 
         if (Time.time >= immortal) 
         {
             immortal = 0;
             animator.Play("Move");
         }
     }
     
     void Acc()
     {
         if(speed < maxSpeed)
             speed += 1f;
         else
             CancelInvoke("Acc");

             rb.velocity = new Vector2 (rb.velocity.x, speed);
     }
 
     void Jump (int strength)
     {
         canJump = false;
         rb.isKinematic = false;
         rb.AddForce (new Vector2 (direction*strength, 0), ForceMode2D.Impulse);
     }
 
     void OnCollisionStay2D(Collision2D collision)
     {    
         canJump = true;
         //rb.isKinematic = true;
     }
             
     void OnTriggerEnter2D (Collider2D collider)
     {
         if (game.gameOver == false) 
         {
             if (collider.tag == "Count Points" && scorePoints)
                 game.score++;
     
             //if (collider.tag == "Obstacle" && immortal == 0)
             //    Camera.main.SendMessage ("Over");
             
             //if(collider.tag == "Meteor" && immortal == 0 && shieldIsActive == false)
             //    Camera.main.SendMessage ("Over");
         }
     }
 
     void Flip ()
     {
         if (direction == 1) {
             GetComponent<SpriteRenderer> ().sprite = sprite [1];
             direction = -1;
         } else {
             GetComponent<SpriteRenderer> ().sprite = sprite [0];
             direction = 1;
         }
         
     }
 
     void Immortal ()
     {
         immortal = Time.time + 3;
         animator.Play("Immortal");
     }
     
     public void ActivateShield ()
     {
         if (cantUseShield == false) 
         {
             if(Social.localUser.authenticated)
                 GooglePlayGames.PlayGamesPlatform.Instance.IncrementAchievement(GPGSIds.achievement_meteors_pff, 1, (bool success)=>{});
 
             transform.FindChild ("Shield").gameObject.SetActive (true);
             shieldIsActive = true;
             Invoke ("DeactivateShield", 1f);
             CooldownShield ();
         }
     }
 
     public void DeactivateShield ()
     {
         transform.FindChild("Shield").gameObject.SetActive(false);
         shieldIsActive = false;
     }
 
     void CooldownShield ()
     {
         //shieldBtn.GetComponent<Animator>().Play("Cooldown");
         shieldBtn.transform.FindChild("Progress").gameObject.SetActive(true);
         cantUseShield = true;
         Invoke("CooldownShieldEnd", 4f);
     }
 
     void CooldownShieldEnd ()
     {
         //shieldBtn.GetComponent<Animator>().Stop();
         shieldBtn.transform.FindChild("Progress").gameObject.SetActive(false);
         cantUseShield = false;
     }
 }
 

The problem is that sometines the player velocity is turning to zero and the player is stuck in place. I have no idea how to fix it. Here are some screenshots for you to better understand how the game works: