ASP.NET Web Forms Dropdownbox selectedindexchange not firing from modal

Question

Tech: ASP.NET Web Forms with C# code behind.

Problem : I have a DropDownList linked to a SelecteddIndexChange event. On selecting the event will fire and do some stuff.

Here is the aspx code:

<tr id="trTypeOfBusiness" runat="server" visible="true">
    <td>
        What was the purpose or intention for registering the business?
    </td>
    <td>
        <asp:DropDownList runat="server" ID="radBusinessType" AutoPostBack="True" 
            OnSelectedIndexChanged="radBusinessType_SelectedIndexChanged"
            CausesValidation="false">
        <asp:ListItem Text="Select One.." Value="" />
        <asp:ListItem Text="Business for profit" Value="Commercial business" />
        <asp:ListItem Text="NGO, non-profit, community related work"
            Value="Non-Profit Organisation" />
        <asp:ListItem Text="Sole Provider" Value="Sole Provider" Enabled="False" />
        <asp:ListItem Text="Trust" Value="Trust" />
        <asp:ListItem Text="To hold/secure a property (movable and immovable)"
            Value="To hold/secure a property (movable and immovable)" />
        </asp:DropDownList>
        <asp:RequiredFieldValidator ID="RequiredFieldValidator2" ValidationGroup="dirship"
            runat="server" CssClass="ErrorSummary" Display="None" 
            ErrorMessage="Type of business is a required field" Text="*" 
            ControlToValidate="radBusinessType" Enabled="True">
        </asp:RequiredFieldValidator>
    </td>
</tr>

Code Behind:

protected void radBusinessType_SelectedIndexChanged( object sender,EventArgs e )
{
    ApplyRadBusinessTypeChangeRules();
}

The dropdown is in an ascx and is loaded with a Modal on btnAdd click:

        <asp:ModalPopupExtender runat="server" ID="ModalPopupExtenderNext" PopupControlID="PanelAddEdit"
            TargetControlID="btnAdd" BackgroundCssClass="modalBackground2" >
        </asp:ModalPopupExtender>
    </td>
</tr>
<tr>
    <td colspan="2" class="tdRight">
        <asp:Button ID="btnAdd" runat="server" CausesValidation="false" CssClass="Button"
            Text="Add New" OnClick="btnAdd_Click" /><br /><br />
    </td>
</tr>

I have 12963 users on the system. It runs on the company LAN and is hosted on IIS 7. The selectedindexchange event fires for 12923 users. The other 40 are complaining it doesn't react. I have remotely accessed their pc's and can confirm it does not work. If they access the application from another workstation/pc it works.

This makes me to believe that it is something on their pc's that are missing or not working. The issue is on both IE and Chrome. We are not allowed to use any other browser.

Anyone have any idea what it could be?