Unable to get second Checkbox onwards status in gridview in asp.net c#

I am adding Select Checkbox at every row in GridView while loading from database. Below is my aspx page code:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
                Width="100%" onrowcancelingedit="GridView1_RowCancelingEdit" 
                    onrowdatabound="GridView1_RowDataBound" onrowediting="GridView1_RowEditing" 
                    onrowupdating="GridView1_RowUpdating">
                <Columns>
                    <asp:TemplateField HeaderText="Select">
                        <ItemTemplate>
                            <asp:CheckBox ID="cbSelect" runat="server"></asp:CheckBox>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="ID">
                        <ItemTemplate>
                            <asp:Label ID="lblid" runat="server" Text='<%# Eval("ID") %>'>
                            </asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Part Number">
                        <ItemTemplate>
                            <asp:Label ID="lblpn" runat="server" Text='<%# Eval("PartNumber") %>'>
                            </asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Part Desc">
                        <ItemTemplate>
                            <asp:Label ID="lblpd" runat="server" Text='<%# Eval("PartDesc") %>'>
                            </asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                </Columns>
            </asp:GridView>

I want to read the data from the rows which are selected in the respective Checkbox. For this I have done following code in Button Click:

 for (int i = 0; i < GridView1.Rows.Count; i++)
                {
                    GridViewRow dr = GridView1.Rows[i];

                    Label lblID = (Label)dr.FindControl("lblid");
                    ID = Convert.ToInt32(lblID.Text);

                    CheckBox checkBox = (CheckBox)dr.FindControl("cbSelect");
                    if (checkBox.Checked)
                    {
}
}

Now if I select single row in grid then I am able to get the checkbox status Checked for that particular row.

But If I select multiple rows then I am getting only first selected row's checkbox status as checked. From second row I am getting Check status as Unchecked. But I am able to get correct value in lblID label control.

Following is my function to fill gridview:

 public void showgrid(string Location)
    {
        DataTable dt = new DataTable();

        con.Open();
        string strQuery = "";
        SqlDataAdapter sda = new SqlDataAdapter();
        if (chkConsiderPart.Checked)
            strQuery = "select * from InventoryDetails where Status='Stocked' and ToLocation='" + Location + "' and PartNumber='" + ddlPartNumber.SelectedItem.Text + "'";
        else
            strQuery = "select * from InventoryDetails where Status='Stocked' and ToLocation='" + Location + "'";
        SqlCommand cmd = new SqlCommand(strQuery);
        cmd.CommandType = CommandType.Text;
        cmd.Connection = con;
        sda.SelectCommand = cmd;
        sda.Fill(dt);
        GridView1.DataSource = dt;
        GridView1.DataBind();

        if (dt.Rows.Count == 0)
        {
            lblRowsStatus.Text = "No Records found for the selection.";
        }
        else
        {
            lblRowsStatus.Text = "There are " + dt.Rows.Count.ToString() + " Records found for the selection.";
        }
    }

I am only calling it in:

 if (!IsPostBack)
        {  
 showgrid(ddlFromLocation.SelectedItem.Text);
        }

Also there was post-back at certain events of other controls. but even if I removed them same issue is happening.

Note: There is no code written in any of Gridview Events.