two connected comboboxes are not working

I have two comboboxes. When I change value of category combobox, it should automatically change value of size combobox.

private void New_Item_Load(object sender, EventArgs e)
{
    // TODO: This line of code loads data into the 'pitauzDBDataSet.Item' table. You can move, or remove it, as needed.
    this.itemTableAdapter.Fill(this.pitauzDBDataSet.Item);

    cbx_product_category.Items.Add("Pita");
    cbx_product_category.Items.Add("Drinks");
    cbx_product_category.Items.Add("Other Foods");
}

private void cbx_product_category_SelectedIndexChanged(object sender, EventArgs e)
{
    string ItemSelected = cbx_product_category.SelectedIndex.ToString();
    if(ItemSelected == "Pita")
    {
        cbx_product_size.Items.Clear();
        cbx_product_size.Items.Add("Small");
        cbx_product_size.Items.Add("Regular");
        cbx_product_size.Items.Add("Large");
        cbx_product_size.Items.Add("Very-Large");
    }
    if(ItemSelected == "Drinks")
    {
        cbx_product_size.Items.Clear();
        cbx_product_size.Items.Add("0.5L");
        cbx_product_size.Items.Add("1L");
        cbx_product_size.Items.Add("1.5L");
    }
    if (ItemSelected == "Other Foods")
    {
        cbx_product_size.Items.Clear();
        cbx_product_size.Items.Add("Half");
        cbx_product_size.Items.Add("Full");
    }
}

It is not giving any error. Just logic not working.