asp:ImageButton not firing OnClick event- it works randomly

Question

Here is the code. I have also tried using oncommad property but still no luck.

These imagebuttons are the part of asp:repeater.

I am using this for online shop.

<span class="col col-xs-12 col-md-3">
  <asp:RegularExpressionValidator ID="QuantityRegularExpressionValidator" runat="server"
    ValidationExpression="\d+" ControlToValidate="QuantityTextBox" ErrorMessage="*" 
    Display="None"></asp:RegularExpressionValidator>

  <asp:RangeValidator ID="QuantityRangeValidator" runat="server" 
    ErrorMessage="RangeValidator" ControlToValidate="QuantityTextBox" 
    MaximumValue="9999" MinimumValue="1" Display="None"></asp:RangeValidator>

  <asp:ImageButton ID="UpdateItemQuantityImageButton" runat="server" 
    ImageUrl="~/Responsive/custom/images/Shop/refreshItem.png"
    Height="19" OnClick="UpdateItemQuantityImageButton_Click"         
    CssClass="shopCartListButton" />

  <asp:ImageButton ID="DeleteItemImageButton" runat="server" 
    ImageUrl="~/Responsive/custom/images/Shop/deleteItemFromCart.png"
    Height="19" OnClick="DeleteItemImageButton_Click" CssClass="shopCartListButton" />
</span>

Code Behind is given below. I have added the page load and page init event, if that help to find out solution.

Thanks in advance.

        protected void Page_Init(object sender, EventArgs e)
{
    siteId = Master.siteId;

    //check if this page is empty.
    if (Master.pageId == Guid.Empty)
    {
        //this page is unknown, so get the page based on the pageTypeId.
        thisPage = DAL.DataClasses.Page.GetPageForSite(pageTypeId, siteId);
    }

    try
    {
        shopCartId = long.Parse(Session["shopCartId"].ToString());
        thisShopCart = DAL.DataClasses.ShopCart.GetObjectById(shopCartId);
        thisShop = thisShopCart.Shop;
    }
    catch
    {
        //can't get a product.
        //go to the shop front.
        thisShop = DAL.DataClasses.Shop.GetDefaultShopForSite(siteId);
        if (thisShop != null)
        {
            Response.Redirect(DAL.DataClasses.Shop.GetDetailPageURL(thisShop.ShopId, siteId));
        }
        else
        {
            //no shop for this site. redirect to home page.
            Response.Redirect(Master.thisSite.WebsiteURL);
        }
    }

    DisplayContent();
}

protected void Page_Load(object sender, EventArgs e)
{
    TransactionTotalLiteral.Text = DAL.DataClasses.ShopCart.CalculateCartTotal(shopCartId).ToString("C");
}

public void DisplayContent()
{
   // PageHeadingLiteral.Text = thisShop.Name;

    IList<ORM.Shopcartitem> shopCartItemList = DAL.DataClasses.ShopCart.GetItemsInCart(shopCartId);

    if (shopCartItemList.Count() == 0)
    {
        CheckoutButton.Visible = false;
        EmptyLiteral.Visible = true;
    }
    else
    {
        EmptyLiteral.Visible = false;
    }

    ShopCartRepeater.DataSource = shopCartItemList;
    ShopCartRepeater.DataBind();
}

protected void DeleteItemImageButton_Click(object sender, ImageClickEventArgs e)
{
    ImageButton thisImageButton = (ImageButton)sender;
    RepeaterItem thisRepeaterItem = (RepeaterItem)thisImageButton.Parent;
    Repeater thisRepeater = (Repeater)thisRepeaterItem.Parent;

    HiddenField ShopCartItemIdHiddenField = (HiddenField)thisRepeaterItem
      .FindControl("ShopCartItemIdHiddenField");
    TextBox QuantityTextBox = (TextBox)thisRepeaterItem.FindControl("QuantityTextBox");

    long shopCartItemId = long.Parse(ShopCartItemIdHiddenField.Value);
    DAL.DataClasses.ShopCart.DeleteItemFromCart(shopCartItemId);
    TransactionTotalLiteral.Text = DAL.DataClasses.ShopCart.CalculateCartTotal(shopCartId)
      .ToString("C");

    DisplayContent();
}