Inconsistent GetMouseButtonDown Behaviour

Hi everyone! I've been easing my way into learning how to code game dialogue by advancing a simple dialogue stream with a mouse click. However, I'm finding that while my code works some of the time, it frequently does not detect the mouse click. Additionally, sometimes when the mouse click is not detected, Unity seems to store the event and trigger it two (or three) times the next time the mouse is clicked, making the dialogue unreadable.

I have already confirmed that this script is only acting on one gameobject. It is also using Text, not GUIText. I'm fairly new to Coroutines, so I suspect the problem is somewhere in there.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class prologueCode : MonoBehaviour {

     public Text textBox, label;
     public TextAsset dialogue;
     string[] lines;
     int i = 0;
     public cameraFade cam;
     bool firstTime = true;
 
     public void Start()
     {
         if (dialogue != null)
         {
              lines = (dialogue.text.Split('\n'));
         }
     }
 
     public void LateUpdate () {
         if (firstTime)
         {
             cam.SetScreenOverlayColor(new Color(0, 0, 0, 1));
             cam.StartFade(new Color(0,0,0,0), 7);
             firstTime = false;
         }
         StartCoroutine(showDialogue());
     }
 
 
     IEnumerator showDialogue()
     {
         yield return new WaitForSeconds(7);
         textBox.text = lines[i];
         while (i < 9) 
         {
             yield return StartCoroutine(waitClick());
             textBox.text = lines[i];
              i++;
         }
     }
 
     IEnumerator waitClick()
     {
         while (!Input.GetMouseButtonDown(0))
         {
             yield return new WaitForEndOfFrame();
         }
     }
 }

If it helps to know, sometimes the array index manages to exceed the loop limit and triggers an outOfBounds exception.

Thanks for all the help!