Dropdownlist not responding to the "OnSelectedIndexChanged" method.

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I have several dropdownlists on my page. The first works as expected and populated the the datasets for the other two and displays them on the screen.

My issue arises once I make a selection from the 2nd and 3rd dropdownlists. It appears that the onSelectedIndexChanged method is not firing.....

<asp:Label ID="Label3" runat="server" Text="Study Selection"></asp:Label>  <br />    <asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="True" onSelectedIndexChanged="displayDefinitions" ><br />    </asp:DropDownList>          <asp:Label <br />        ID="Label1" runat="server" Text="Row Selection" Visible="False"></asp:Label>  <br />    <asp:DropDownList ID="DropDownList1" runat="server" Visible="false" onSelectedIndexChanged="nextStep"  ><br />    </asp:DropDownList>          <asp:Label <br />        ID="Label2" runat="server" Text="Column Selection" Visible="False"></asp:Label>  <br />    <asp:DropDownList ID="DropDownList2" runat="server" Visible="false" onSelectedIndexChanged="nextStep" ><br />    </asp:DropDownList><br />

DropDownList3 works fine.
DropDownList1 and DropDownList2 do not. I can make a selection, but the underlying code is not called.

I'm sure it's going to be something deceptively simple, but I'm just not seeing it.
Thanks for any help offered.