Unable to uncheck a checkbox at first click

Question

Hi,

I am not able to uncheck a checkbox at first click, on the second click it works fine. I am pretty new to .Net and this is my first time facing it, checking the checkboxes works fine with no issue. I am not able to debug as unchecking the checkbox does not even trigger a page refresh or the event attached to it. This is my code. Please help. 

protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
        {
            //List<string> user_select_weeks_list = new List<string>();
            AllLists create1 = new AllLists();

            for (int i = 0; i < rptItemsInCart.Items.Count; i++)
            {
                CheckBox ref_to_checkBox = (CheckBox)rptItemsInCart.Items[i].FindControl("Checkbox1");
                if (ref_to_checkBox.Checked)
                {
                    Ok_prompt_to_upload_Button.Visible = true;
                    ref_to_checkBox.ID = "Week" + (i + 1);
                    Label2.Text = ref_to_checkBox.ID;
                    create1.user_select_weeks_list.Add(ref_to_checkBox.ID);

                    //user_select_weeks_list.Add(ref_to_checkBox.ID);
                }
                else
                {
                    create1.user_select_weeks_list.Remove(ref_to_checkBox.ID);
                }
            }
            selected_weeks_repeater.DataSource = create1.user_select_weeks_list;
            selected_weeks_repeater.DataBind();

        }

public class AllLists
   {
       public List<string> user_select_weeks_list { get; set; }

       public AllLists()
       {
           user_select_weeks_list = new List<string>();
       }
   }

I don't have anything in my page load, am I missing something.