Unable to spawn non-player object (with rigidbody2D) using NetworkServer.Spawn() method

I am new to Unity and trying to create a multiplayer Pong game. I was able to spawn Player objects using Network Manager (auto-spawn) and Network Transform and Network Identity components, but when I try to spawn 'Ball' object (non-player game object), using NetworkServer.Spawn() method, then its not working. Note that, I have added Network Identity and Network Transform components to Ball gameobject. Also, I have registered my Ball prefab to Network Manager's Spawnable Prefabs slot.

Below is my code for spawning Ball gameobject -

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.Networking;
3. using System.Collections;
4.  
5. public class BallSpawner : NetworkBehaviour {
6.  
7.     [SerializeField]
8.     private GameObject ballPrefab;
9.  
10.     GameObject ball;
11.     NetworkHash128 ballAssetId;
12.  
13.     void Start()
14.     {
15.         ballAssetId = ballPrefab.GetComponent<NetworkIdentity>().assetId;
16.         ball = Instantiate(ballPrefab, Vector3.zero, Quaternion.identity) as GameObject;
17.     }
18.  
19.     public override void OnStartServer()
20.     {
21.         NetworkServer.Spawn(ball);
22.         base.OnStartServer();
23.     }
24. }
25.  

Any help is highly appreciated.
Thanks.