Task list item not moving back to 'completed' list holder

var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementById("add-button"); //first button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); //incomplete-tasks
var completedTasksHolder = document.getElementById("completed-tasks"); //completed-tasks

//New task list item
var createNewTaskElement = function(taskString) {
    var listItem = document.createElement("li");
    var checkBox = document.createElement("input");
    var label = document.createElement("label");
    var editInput = document.createElement("input");
    var editButton = document.createElement("button");
    var deleteButton = document.createElement("button");

    //Each element needs modifying

    checkBox.type = "checkbox";
    editInput.type = "text";

    editButton.innerText = "Edit";
    editButton.className = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.className = "delete";

    label.innerText = taskString;

    //each element needs appending
    listItem.appendChild(checkBox);
    listItem.appendChild(label);
    listItem.appendChild(editInput);
    listItem.appendChild(editButton);
    listItem.appendChild(deleteButton);

    return listItem;    
};

//Delete 'add item' inner-text
var deleteItemText = function () {
    taskInput.value = "";
};

var ChangeAddButtonClass = function () {
    if(taskInput.value == "Input text before adding" || taskInput.value == "") {
        addButton.classList.add("error-hover"); 
    }else {
        addButton.classList.remove("error-hover");  
    }
};

//Add a new task
var addTask = function() {
    //Create a new list item with the text from #new-task:
    if(taskInput.value) {
        if(taskInput.value != "Input text before adding") {
            var listItem = createNewTaskElement(taskInput.value);
            //Append listItem to the incompleteTasksHolder
            incompleteTasksHolder.appendChild(listItem);
            bindTaskEvents(listItem, taskCompleted);
        }   
    taskInput.value = "";
    }
};

//Edit an existing task
var editTask = function() {
    var listItem = this.parentNode;
    var editInput = listItem.querySelector("input[type=text]");
    var label = listItem.querySelector("label");
    var editButton = listItem.querySelector("button.edit");
    var containsClass = listItem.classList.contains("editMode");

        //If the class of the parent is .editMode
        if(containsClass) {
            //Switch from .editMode
            //label text become the input's value
            editButton.innerText = "Edit"
            label.innerText = editInput.value;
        } else {
            //Switch to .editMode
            //input value becomes the label's text
            editButton.innerText = "Save"
            editInput.value = label.innerText;  
        }

        //Toggle .editMode on the listItem
        listItem.classList.toggle("editMode");
};

//Delete an existing task
var deleteTask = function() {
    var listItem = this.parentNode;
    var ul = listItem.parentNode;
    //Remove the parent list item from the ul
    ul.removeChild(listItem);
};

//Mark a task as complete
var taskCompleted = function() { 
    //Append the task list item to the #completed-tasks
    var listItem = this.parentNode;
    completedTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskIncomplete);
};

//Mark a task as incomplete
var taskIncomplete = function() {
    //When the checkbox is unchecked append to the #incomplete-tasks
    var listItem = this.parentNode;
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
};

var bindTaskEvents = function (taskListItem, checkBoxEventHandler) {
    //select listItem's children
    var checkBox = taskListItem.querySelector("input[type=checkbox]");
    var editButton = taskListItem.querySelector("button.edit");
    var deleteButton = taskListItem.querySelector("button.delete");

        //bind editTask to edit button.
        editButton.addEventListener("click", editTask);

        //bind deleteTask to delete button
        deleteButton.addEventListener("click", deleteTask);

        //bind checkBoxEventHandler to checkbox;
        checkBox.addEventListener("change", checkBoxEventHandler);
};

var ajaxRequest = function(){
    console.log("AJAX Request")
}

//set the click handler to the addTask function
taskInput.addEventListener("focus", deleteItemText);
addButton.addEventListener("mouseover", ChangeAddButtonClass);
addButton.addEventListener("click", addTask);
addButton.addEventListener("click", ajaxRequest);

//cycle over the incompleteTasksHolder ul list items
for (i = 0; i < incompleteTasksHolder.children.length; i++ ) {
    //for each list item
        //bind events to the list item's children (taskCompleted)
        bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}


//cycle over the completedTasksHolder
for (i = 0; i < completedTasksHolder.children.length; i++) {
    //for each list item
        //bind events to the list item's children (taskIncomplete)
        bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}