エフェクトの生成について

Unityの銃弾発射エフェクトを作っているのですが、2回に1回ほどシーンを再生すると正常にエフェクトオブジェクトが生成されない時があります。
このシーン上に関係のあるスクリプトは下記の3つでシーン上では
RotateToMouseスクリプトがSphere
SpawnスクリプトがScene
ProjectileMoveスクリプトがプレフハブ化した問題のエフェクトについています。
プレハブ化したエフェクトにはBoxcolliderもついています。

おそらく、オブジェクトの生成を指示しているSpawnに問題があると思っているのですがどなたか分かりますでしょうか？

public class RotateToMouse : MonoBehaviour {

	public Camera cam;
	public float maximumLenght;

	private Ray rayMouse;
	private Vector3 pos;
	private Vector3 direction;
	private Quaternion rotation;

	void Update () {
		if (cam != null) {
			RaycastHit hit;
			var mousePos = Input.mousePosition;
			rayMouse = cam.ScreenPointToRay (mousePos);
			if (Physics.Raycast (rayMouse.origin, rayMouse.direction, out hit, maximumLenght)) {
				RotateToMouseDirection (gameObject, hit.point);
			} else {
				var pos = rayMouse.GetPoint (maximumLenght);
				RotateToMouseDirection (gameObject, pos);
			}
		} else {
			Debug.Log ("No Camera");
		}		
	}

	void RotateToMouseDirection (GameObject obj, Vector3 destination){
		direction = destination - obj.transform.position;
		rotation = Quaternion.LookRotation (direction);
		obj.transform.localRotation = Quaternion.Lerp (obj.transform.rotation, rotation, 1);
	}

	public Quaternion GetRotation (){
		return rotation;
	}
}

public class ProjectileMove : MonoBehaviour {

	public float speed;
	public float fireRate;
	public GameObject muzzlePrefab;
	public GameObject hitPrefab;

	void Start () {
		if (muzzlePrefab != null) {
			var muzzleVFX = Instantiate (muzzlePrefab, transform.position, Quaternion.identity);
			muzzleVFX.transform.forward = gameObject.transform.forward;
			var psMuzzle = muzzleVFX.GetComponent<ParticleSystem> ();
			if (psMuzzle != null)
				Destroy (muzzleVFX, psMuzzle.main.duration);
			else {
				var psChild = muzzleVFX.transform.GetChild (0).GetComponent<ParticleSystem> ();
				Destroy (muzzleVFX, psChild.main.duration);
			}
		}
	}

	void Update () {
		if (speed != 0) {
			transform.position += transform.forward * (speed * Time.deltaTime);
		}else{
			Debug.Log("No Speed");
		}
	}

	void OnCollisionEnter (Collision co){
		speed = 0;

		ContactPoint contact = co.contacts [0];
		Quaternion rot = Quaternion.FromToRotation (Vector3.up, contact.normal);
		Vector3 pos = contact.point;

		if (hitPrefab != null) {
			var hitVFX = Instantiate (hitPrefab, pos, rot);
			var psHit = hitVFX.GetComponent<ParticleSystem> ();
			if (psHit != null)
				Destroy (hitVFX, psHit.main.duration);
			else {
				var psChild = hitVFX.transform.GetChild (0).GetComponent<ParticleSystem> ();
				Destroy (hitVFX, psChild.main.duration);
			}
		}

		Destroy (gameObject);
	}
}

public class Spawn : MonoBehaviour {

	public GameObject firePoint;
	public List<GameObject> vfx = new List<GameObject> ();
	public RotateToMouse rotateToMouse;

	private GameObject effectToSpawn;
	private float timeToFire = 0;

	void Start () {
		effectToSpawn = vfx [0];
	}

	void Update () {
		if(Input.GetMouseButton (0) && Time.time >= timeToFire){
			timeToFire = Time.time + 1 / effectToSpawn.GetComponent<ProjectileMove> ().fireRate;
			SpawnVFX ();
		}
	}

	void SpawnVFX (){
		GameObject vfx;

		if (firePoint != null) {
			vfx = Instantiate (effectToSpawn, firePoint.transform.position, Quaternion.identity);
			if (rotateToMouse != null) {
				vfx.transform.localRotation = rotateToMouse.GetRotation ();
			}
		} else {
			Debug.Log ("No Fire Point");
		}
	}
}