Invoke is not working consistently

I have a problem with this script. What it is supposed to do, is when the timer (timeLeft) = 0, it is supposed to change the text (countdownText) to "Rush!", then after 15 seconds, change the timer to 11 seconds. It works flawlessly the first time through, but then, once 11 seconds have passed after the first time, it doesn't show "Rush". Instead it goes straight back to 11 seconds, and counts down from there again. Does anyone know why this is? Here is the code:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class Countdown : MonoBehaviour {
 
     public float timeLeft = 60;
     public AudioClip rushIncomingClip;
     public Spawner spawner;
     public float maxSpawnDelaySetter = 4f;
 
     private Text countdownText;
     private Color textColor;
     private Color defaultTextColor;
     private AudioSource source;
     private bool bellNotRung = true;
     private bool timerHasBeenReset = false;
 
     // Use this for initialization
     void Start () {
         countdownText = GetComponent<Text>();
         defaultTextColor = countdownText.color;
         source = GetComponent<AudioSource>();
     }
     
     // Update is called once per frame
     void Update () {
         countdownText.text = Mathf.RoundToInt(timeLeft).ToString();
         countdownText.color = textColor;
         timeLeft -= Time.deltaTime;
 
         if (timeLeft <= 10.49) {
             textColor = Color.red;
         } else {
             textColor = defaultTextColor;
         }
 
         if (timeLeft < 0) {
             if (bellNotRung) {
                 source.PlayOneShot(rushIncomingClip, 1f);
                 bellNotRung = false;
             }
             countdownText.text = "Rush!";
             Rush();
             timerHasBeenReset = false;
             Invoke("TimerReset", 15f);
         }
     }
 
     void TimerReset() {
         if (timerHasBeenReset == false) { 
             timeLeft = 11f;
             timerHasBeenReset = true;
         } 
         bellNotRung = true;
         maxSpawnDelaySetter = 4f;
     }
 
     void Rush() {
         maxSpawnDelaySetter = 0.6f;
     }
 }