Drop Down List

Ovo je kod aspx:

<asp:UpdatePanel runat ="server">
<ContentTemplate>
<div id="popupdiv" style="display: none; position: fixed;
top: 50%;
left: 50%;
margin-top: -50px;
margin-left: -100px;
width:500px;
height:600px;
transform: translate(-50%, -50%);
border: 1px solid #000000;
background-color: white">


<table style ="height:40px;background-color: #efefef;width:100%">
<tr><td style="text-align:center;vertical-align:middle;font-family:Calibri;font-weight:bold;font-size:large">UNOS OTPREMNICE</td></tr><tr><td>&nbsp;</td></tr>
</table><br />
<table style ="margin-left:30px;">
<tr>
<td>Grupa materijala</td>
</tr>
<tr>
<td>
<asp:DropDownList ID="ddlGrupaMat" runat="server" OnSelectedIndexChanged ="gpMat_IndexChanged" AutoPostBack ="true">
<%--<asp:ListItem Value ="{{:Name}}">
</asp:ListItem>--%>
</asp:DropDownList>
</td>
</tr><tr><td>&nbsp;</td></tr>
<tr>
<td>Materijal</td>
</tr>
<tr>
<td><asp:DropDownList ID="ddlMaterijal" runat="server" /></td>
</tr><tr><td>&nbsp;</td></tr>

 
</table>
</div>

</ContentTemplate> <Triggers>
<asp:AsyncPostBackTrigger ControlID="ddlGrupaMat" EventName="SelectedIndexChanged" />
</Triggers></asp:UpdatePanel>

ovo je kod vb.net:

Protected Sub gpMat_IndexChanged(ByVal sender As Object, ByVal e As EventArgs)

ddlMaterijal.DataSource = uma.dohvatiListuWithParam(conn, dohvatiOfficeMaterijalDDL, "@GroupID", ddlGrupaMat.SelectedValue)
ddlMaterijal.DataValueField = "MaterialID"
ddlMaterijal.DataTextField = "Materijal"
ddlMaterijal.DataBind()

End Sub

Ali ddlMaterijal drop down lista se ne napuni. Zašto?