coroutine doesn't work the second time

when i run my Game, the first Time i talk to my npc my code works normal. but when i try to talk to him a second time, it skips the dialogue. Can anyone help me?

My code: using System.Collections; using System.Collections.Generic; using System.Linq; using UnityEngine; using UnityEngine.UI;

 namespace project
 {
     public class NPC : MonoBehaviour
     {
 //checks if the player talks the first Time with the npc
         public bool hasSpoken = false;
 //The dialogue stuff
         public GameObject dialogBox;
         public Text dialogText;
 //a hint, wich says you wich button to press
         public Text hintText;
         public string hint;
 //the name of the npc
         public Text NPCname;
         public string NameText;
 //checks, if the player is near the npc
         public bool PlayerInRange;
 //more stuff for the hint
         public GameObject hintObject;
 //movment script
         public PlayerMovment movment;
 //wich text to show
         private string dialog;
 //wich quest the player is working on
         public int currentBook;
 //wich choice the player made in the quest
         public int choiceMade;
 //quest script
         public book book;
         
         
 
         private void Start()
         {
             movment.AllowedToMove = true;
             
             NPCname.text = NameText;
             PlayerInRange = false;
             choiceMade = 0;
         }
 
         private void Update()
         {
             if (PlayerInRange && Input.GetKeyDown(KeyCode.Space))
             {
                 
                 dialogBox.SetActive(true);
                 hintObject.SetActive(false);
                 movment.AllowedToMove = false;
 
                 StartCoroutine(DialogueSystem());
                 
             }
         }
 
         public void OnTriggerEnter2D(Collider2D collision)
         {
 
             hintObject.SetActive(true);
             hintText.text = hint;
             PlayerInRange = true;
             
         }
 
         public void OnTriggerExit2D(Collider2D collision)
         {
             hintObject.SetActive(false);
             PlayerInRange = false;
         }
 //dialogue
        IEnumerator DialogueSystem()
         {
             if (!hasSpoken)
             {
                 yield return waitForKeyPress(KeyCode.Space);
                 NextSentence("(dialogue).");
                 yield return waitForKeyPress(KeyCode.Space);
                 NextSentence("(dialogue)");
                 yield return waitForKeyPress(KeyCode.Space);
                 dialogBox.SetActive(false);
                 hintObject.SetActive(true);
                 movment.AllowedToMove = true;
                 hasSpoken = true;
                 currentBook = 1;
                 yield return null;
             }
             else
             {
                 if(currentBook == 1)
                 {
                     if (choiceMade == 0)
                     {
 //this part doesn't work
                         yield return waitForKeyPress(KeyCode.Space);
                         NextSentence("(Dialogue)");
                         yield return waitForKeyPress(KeyCode.Space);
                         dialogBox.SetActive(false);
                         hintObject.SetActive(true);
                         movment.AllowedToMove = true;
                     
                     }
                 }
             }
 
         }
 //shows the next sentence
         public void NextSentence(string sentence)
         {
             
             sentence = sentence.Replace("@", System.Environment.NewLine);
             dialogText.text = sentence;
             
         }
 
 //waits for the key to press
         public IEnumerator waitForKeyPress(KeyCode key)
         {
             
             bool done = false;
             
             while (!done) 
             {
                 if (Input.GetKeyDown(key))
                 {
                     done = true; 
                 }
                 yield return null;
             }
 
             
         }
 
 
 
     }
 }