Problem in 2d sprite creation

I want to create sprites that fall from random top positions or x and y. I have created a square object I mean sprite and add this code:

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

public class RandomFall : MonoBehaviour

    public GameObject spritePrefab;
    public float spawnInterval = 1f;
    public float spawnRadius = 5f;
    public float fallSpeed = 2f;

    private float timer;

    // Start is called before the first frame update
    void Start()

    // Update is called once per frame
    void Update()
        timer += Time.deltaTime;

        if(timer >= spawnInterval){
            timer = 0f;

    private void spawnSprite() {
        float spawnX = Random.Range(-spawnRadius, spawnRadius);
        float spawnY = transform.position.y + spawnRadius;
        Vector3 spawnPosition = new Vector3(spawnX, spawnY, 0f);
        GameObject sprite = Instantiate(spritePrefab, spawnPosition, Quaternion.identity);
        Rigidbody2D rb = sprite.GetComponent<Rigidbody2D>();
        rb.velocity = Vector2.down * fallSpeed;

I have added Rigidbody2D and yes it falls but onetime and one sprite. Where is the problem?