The selectedIndexChanged for dropdown doesn't fire in updPnl

Question

 <table class="artmaterialsupplies">
<tr><td style="text-align:center">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<br />
<table style="width:100%" cellpadding="0" border=0>
<tr><td colspan="3" style="text-align:left;font-size:18px; font-weight:bold;background-color:#ffffff; color:#249c98">
<asp:Label ID="lblBrandName" runat="server" Text=''></asp:Label>
<asp:HiddenField ID="hdnProductChange" runat="server" Value='' />
<asp:HiddenField ID="hdnImageURL" runat="server" Value='' />
<asp:HiddenField ID="hdnProductName" runat="server" Value='' />
</td></tr>
<tr>
<td style="vertical-align:top;">
<asp:Image ID="imgArtMaterials" runat="server" />
</td>
<td style="vertical-align:top;width:50px;">
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always">
<ContentTemplate>
<asp:Panel ID="pnlColour" runat="server" style="width:45px;height:45px;border:1px solid #000;" >
</asp:Panel>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="drpAvailableColours" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>

</td>
<td style="text-align:left;vertical-align:top;">
<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Always">
<ContentTemplate>

<asp:Label ID="lblProductName" runat="server" Text="" style="font-weight:bold;" ></asp:Label><br />
<asp:Label ID="lblProductCode" runat="server" Text=""></asp:Label><br />
<span style="color:#ff0000;font-size:16px;">Price:&nbsp;<asp:Label ID="lblPrice" runat="server" Text=""></asp:Label>AED</span>

<br /><br />
Available Sizes<br />
<asp:DropDownList ID="drpSizes" AutoPostBack="false" runat="server" >
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="drpAvailableColours" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<br /><br />
Available Colours<br />

<asp:DropDownList ID="drpAvailableColours" runat="server" AutoPostBack="True" onselectedindexchanged="drpAvailableColours_SelectedIndexChanged" >
</asp:DropDownList>
<br /><br />
Quantity&nbsp;&nbsp;<asp:ListBox ID="ListBox1" runat="server" Rows="1">
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
<asp:ListItem>4</asp:ListItem>
<asp:ListItem>5</asp:ListItem></asp:ListBox>
<br /><br />
<asp:ImageButton ID="ImgAddToCart" runat="server" ImageUrl="~/Images/AddToCart.png" />
</td>

</tr>
</table>


</td>
</tr>
</table>