Gets stuck on WaitForSeconds

I'm having a problem with Coroutines and yield. When the ball passes the goal lines the score updates and then a new ball i supposed to be spawned after 1.5 sec. But I get stuck on yield return new WaitForSeconds(1.5f); in Pong.cs. I've read the docs but I can still not figure out whats the problem.

The score is added but no ball is spawned.

Updates:

• If I comment out yield return new WaitForSeconds(1.5f); the code works, but of course doesn't wait.

• I'm new to Unity and C# so don't trust my code as it was your own :)

Ball.cs attached to BallPrefab

 using UnityEngine;
 using System.Collections;
 
 public class Ball : MonoBehaviour {
     
     private static int TOP, BOT;
     private static Pong PONG;
     
     public float maxVelocity = 3;
     public float minVelocity = 0.1f;
     
     void Awake () {
         PONG = Pong.INSTANCE;
         TOP = ((int)Pong.TOP.z) + 2;
         BOT = ((int)Pong.BOT.z) - 2;
         rigidbody.velocity = new Vector3(0, 0, -7);
     }
     
     // Update is called once per frame
     void Update () {
         
         //Make sure we stay between the MAX and MIN speed.
         float totalVelocity = Vector3.Magnitude(rigidbody.velocity);
         if(totalVelocity>maxVelocity) {
             float tooHard = totalVelocity / maxVelocity;
             rigidbody.velocity /= tooHard;
         } else if (totalVelocity < minVelocity) {
             float tooSlowRate = totalVelocity / minVelocity;
             rigidbody.velocity /= tooSlowRate;
         }
         
         if(transform.position.z <= BOT) {
             Destroy(gameObject);
             StartCoroutine(PONG.Goal(1));
         } else if(transform.position.z >= TOP) {
             Destroy(gameObject);
             StartCoroutine(PONG.Goal(2));
         }
     }
 }

Pong.cs attached to MainCamera

 using UnityEngine;
 using System.Collections;
 using System.Threading;
 using System.Diagnostics;
 
 public enum PongGameState { playing, won, lost };
 
 public class Pong : MonoBehaviour {
     
     public Transform ballPrefab;
     public static Pong INSTANCE;
     
     public static Vector3 TOP = new Vector3(10, 0, 15);
     public static Vector3 BOT = new Vector3(10, 0, 5);
     public static Vector3 LEFT = new Vector3(5, 0, 10);
     public static Vector3 RIGHT = new Vector3(15, 0, 10);
     
     private PongGameState gameState;
     
     void Awake() {
         INSTANCE = this;
         gameState = PongGameState.playing;
         Time.timeScale = 1.0f;
         SpawnBall();
     }
     
     private void SpawnBall() {
         print("spawnball");
         Instantiate(ballPrefab, new Vector3(10.0f, 0.0f, 10.0f), Quaternion.identity);
     }
     
     public IEnumerator Goal(int player){
         print("in goal");
         Scores.AddPoint(player);
         yield return new WaitForSeconds(1.5f);
         print("in goal");
         SpawnBall();
     }
 }