my 2d character won't move when i hold any movement key?

i started recently learning the unity basics and programmin (about 4 weeks ago) and i'm enjoying it very much, usually whenver i'm stuck i search on the internet and i find a solution everytime but this time i didn't find anyone encountered the same problem as me so here my problem . i made a movement script to my 2d character using c# , it worked fine but to make my character move i have to click the key everytime and if i keep holding it he won't move .

 public class NinjaMovement : MonoBehaviour
 {
     public float speed = 3f;
 
     public Rigidbody2D rb;
 
     public float jump = 2f;
 
 
     void awake ()
     {
         rb = GetComponent<Rigidbody2D>();
     }
     
 
     // Update is called once per frame
     void FixedUpdate()
     {
         // ...moving left
        if (Input.GetKeyDown(KeyCode.LeftArrow))
         {
             Debug.Log(rb);
             rb.velocity = new Vector2(speed * -1,rb.velocity.y);
         }
         //...moving right
         if (Input.GetKeyDown(KeyCode.RightArrow))
         {
             
             Debug.Log(rb);
             rb.velocity = new Vector2(speed , rb.velocity.y);
          }
         //...jumping
         if (Input.GetKeyDown(KeyCode.UpArrow))
         {
             rb.velocity = new Vector2(jump, rb.velocity.x);
         }
     }
 }