Issue saving gridview checkbox checked items from multiple pages while paging

I have an issue saving gridview checkbox checked items values while paging. I am unable to retrieve the value items for which checkbox is checked. Appriciate any help in this regard

 protected void BtnSaveAuthorization_Click(object sender, EventArgs e)
    {
        SqlCommand cmd = new SqlCommand(); 
        try {

            string pageId = string.Format("Page{0}", GVRegisterNewCarton.PageIndex);
            bool[] selectedCheckboxes = ViewState[pageId] as bool[];
            if (selectedCheckboxes != null)
            {
               for (int i = 0; i < GVRegisterNewCarton.Rows.Count; i++)
                {                      

                    string UserName = ((Label)GVRegisterNewCarton.Rows[i].FindControl("UserName")).Text;

                    string DeptCode = ((Label)GVRegisterNewCarton.Rows[i].FindControl("DeptCode")).Text;                        

                    const string SQLFilePermoutDelivery = "INSERT INTO Test1(UserName, DeptCode) VALUES(@UserName, @DeptCode)";

                    SqlCommand cmdFilePermoutDelivery = new SqlCommand(SQLFilePermoutDelivery, con1);
                    cmdFilePermoutDelivery.Parameters.Add("@UserName", SqlDbType.VarChar);
                    cmdFilePermoutDelivery.Parameters["@UserName"].Value = UserName;
                    cmdFilePermoutDelivery.Parameters.Add("@DeptCode", SqlDbType.VarChar);
                    cmdFilePermoutDelivery.Parameters["@DeptCode"].Value = DeptCode;
                    con1.Open();
                    cmdFilePermoutDelivery.ExecuteNonQuery();
                    con1.Close();


                    BindData();

                }
            }

            ScriptManager.RegisterClientScriptBlock(Page, Page.GetType(), Guid.NewGuid().ToString(), "alert('Success');", true);

            BindData(); 
        }
        catch (Exception ex) 
        { 
            ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "alert('Error occured : " + ex.Message.ToString() + "');", true);

        }        

}