Textbox value in gridview in updatepanel - updatepanel

Question

trying to get value from textbox called tbAantalgv in a gridview within an updatepanel after button click. standard value after databind for tbAantalgv = 0, but when i change the textbox value to 1, the textbox value will NOT change.
For Each gvr As GridViewRow In gvOnderdelenLos.Rows
If gvr.RowType = DataControlRowType.DataRow Then
Dim lblCode As Label = gvr.FindControl("lblCode")
Dim lblArtikel As Label = gvr.FindControl("lblArtikel")
Dim tbAantalgv As TextBox = gvr.FindControl("tbAantalgv")
End If
Next
<asp:UpdatePanel ID="pnlOnderdelenLos" runat="server" style="width:95%; border:solid 3px white; display:none;" UpdateMode="Conditional" >
<ContentTemplate>
<asp:ImageButton ID="imgOnderdelenLosToevoegen" runat="server" OnClientClick="imgOnderdelenLosToevoegen()" ImageUrl="~/Afbeeldingen/Icons/table_row_insert.png"
style="vertical-align:text-top; float:right; margin-right:2%" />
<asp:SqlDataSource ID="dsOnderdelenLos" runat="server" ConnectionString="<%$ ConnectionStrings:AftersalesConnectionString %>"
SelectCommand="EXEC spAppPnlOnderdelenLos">
</asp:SqlDataSource>
<asp:GridView ID="gvOnderdelenLos" runat="server" DataSourceID="dsOnderdelenLos" AllowSorting="False" PagerSettings-PageButtonCount="20" ShowHeader="false"
AllowEditing="True" AutoGenerateColumns="False" CellPadding="3" GridLines="Both" AllowPaging="False" style="width:100%; margin-left:2px;">
<HeaderStyle CssClass="Gridview" />
<Columns>
<asp:TemplateField HeaderText="Artikelnummer" SortExpression="Code" ItemStyle-Width="25%" >
<ItemTemplate>
<asp:Label ID="lblCode" runat="server" Text='<%# Bind("Code")%>' ></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Artikel" SortExpression="Omschrijving" ItemStyle-Width="25%">
<ItemTemplate>
<asp:Label id="lblArtikel" runat="server" Text='<%# Bind("Omschrijving")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Aantal" SortExpression="Aantal" ItemStyle-Width="25%">
<ItemTemplate>
<asp:TextBox id="tbAantalgv" runat="server" Width="95%" text='<%# Bind("Aantal")%>'></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Kostenplaats" SortExpression="Kostenplaats" ItemStyle-Width="25%">
<ItemTemplate>
<asp:DropDownList ID="dlKostenplaats" runat="server" Width="100%">
<asp:ListItem Text="Werkplaats"></asp:ListItem>
<asp:ListItem Text="Verkoop"></asp:ListItem>
</asp:DropDownList>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
<asp:Button ID="btnOnderdelenLosToevoegen" runat="server" OnClick="btnOnderdelenLosToevoegen_click" style="display:none;"/>
I do not understand what the issue can be.
Thanks in advance.
Frank