ASP.NET Label Inside UpdatePanel Not Updating on Selected Index Changed - asp.net

Question

I am new to ASP.NET and I'm trying to get a Label to update with some information that is grabbed when I hit On selected index changed. The On selected index changed function is called and returns just fine (I've debugged and stepped through the whole thing). The only thing that doesn't work is where I set the text of the Labels I'm trying to update.
This is the function that gets called on the On selected index changed click:
protected void OnClosetIndexChanged(object sender, EventArgs e)
{
{
UpdatePanel updatePanel1 = Row.FindControl("UpdatePanel1") as UpdatePanel;
Label oldSourceQuantity = (Label)Row.FindControl("lblQuantity");
oldSourceQuantity.Text = "0";//Trying to force a value
updatePanel1.Update();
}
}
I know it goes into the if and tries to set the text but nothing happens on the client side.
This is the UpdatePanel I have:
<asp:TemplateField>
<ItemTemplate>
<tr>
<td colspan="100%" style="background:#F5F5F5" >
<div id="div<%# Eval("componente_id") %>" style="overflow:auto; display:none; position: relative; left: 15px; overflow: auto">
<div class="ExpandTableHeader">
Cambiar la cantidad
</div>
<div class="body">
<label for="validationOfTypeID">Armario:</label>
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always" ChildrenAsTriggers="true">
<ContentTemplate>
<asp:DropDownList ID="drCloset" AppendDataBoundItems="True" runat="server" Width="20%" Height="30px" AutoPostBack="true" OnSelectedIndexChanged = "OnClosetIndexChanged"></asp:DropDownList>
<br/>
<label for="validationOfTypeID" visible="false" >cajon</label> <br/>
<asp:DropDownList ID = "drDrawer" AutoPostBack="true" runat="server" Width="20%" Height="30px" >
</asp:DropDownList>
<asp:Label ID="lblQuantity" runat="server" Text=""></asp:Label>
</ContentTemplate>
<Triggers>
<asp:AsyncPostbackTrigger ControlID="drCloset" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<label for="validationOfTypeID"></label>
<asp:DropDownList Height="30px" ID="drOperation" runat="server" >
<asp:ListItem>+</asp:ListItem>
<asp:ListItem>-</asp:ListItem>
</asp:DropDownList>
<asp:TextBox width="50px" ID="txtChangeQuantity" runat="server" TextMode="Number" min="0" step="1" Value="0" ></asp:TextBox>
<asp:Label ID="lblTotal" runat="server" Text=""></asp:Label>
<br/>
</br>
<asp:Button class="btn btn-primary" ID="btnConfirmPurchases" runat="server" Text="Validar" AutoPostback="true" width="20%" />
</div>
<asp:DetailsView id="DetailsView1" DataKeyNames="componente_id" Runat="server" Width="300px" Font-Names="Calibri"/>
</td>
</tr>
</ItemTemplate>
</asp:TemplateField>