2d GameObject is sticked to block in air

Hi,

I am creating running game, where player is pushed forward. But when it collides in air, it doesn't fall down but stay on position in air.

Here is my movement script: public class Player : MonoBehaviour {

     public float maxSpeed = 10f;
     public float minSpeed = 5f;
     public float addValue= 0.1f;
     public float speed;
     public float jumpForce;
 
 
     public bool grounded = false;
     public Transform groundCheck;
     private float groundRadius = 0.1f;
     public LayerMask whatIsGround;
 
     void Start () {
         speed = 0f;
     }
 
 
     void FixedUpdate () {
         grounded = Physics2D.OverlapCircle (groundCheck.position, groundRadius, whatIsGround);
         move ();
 
     }
 
 
     void move(){
         if (speed < minSpeed || (speed < maxSpeed && !grounded) )
             speed += addValue;
         if (speed > minSpeed && grounded) {
             speed -= addValue;        
         }
 
         this.rigidbody2D.velocity = new Vector2(speed, this.rigidbody2D.velocity.y);
     }
 
     void Update(){
         if(grounded && Input.GetKeyDown ("space")){
             rigidbody2D.AddForce (new Vector2 (0, jumpForce));
         }
     }

I would really appreciate, if someone could help me with this.

Best regards, Jakub Peschel