ASP.Net onclick not firing when checkbox is checked - asp.net

Question

I'm building an asp.net/C# application that has several true/false questions, along with some multiple choice questions, in it. I've written code for this, and for some reason the onclick() event is not triggering when a checkbox is clicked. This is confirmed working in another app ; I don't understand why it isn't working here. Please see the relevant code below. I've sat this code side-by-side with the known working code, and can't find any anomalies. Does anyone have an idea why this isn't working?
<script type="text/javascript">
function ChkQuestion1(obj) {
var chk1a = document.getElementById("<%=chkFalse1.ClientID %>");
var chk1b = document.getElementById("<%=chkTrue1.ClientID %>");
if (obj.checked == true) {
chk1a.checked = false;
chk1b.checked = false;
obj.checked = true;
}
}
function ClientValidation1(source, arguments) {
var chka = document.getElementById("<%=chkFalse1.ClientID%>");
var chkb = document.getElementById("<%=chkTrue1.ClientID%>");
if (chka.checked == false && chkb.checked == false) {
arguments.IsValid = false;
}
}
</script>
<tr>
<td valign="top">1.</td>
<td width="70px" valign="top">
<asp:CheckBox ID="chkTrue1" runat="server" Text=" " onclick="ChkQuestion1(this)" />
<asp:Label ID="lblTrue1" runat="server" Text="True"></asp:Label></td>
<td width="90px" valign="top">
<asp:CheckBox ID="chkFalse1" runat="server" Text=" " onclick="ChkQuestion1(this)" />
<asp:Label ID="lblFalse1" runat="server" Text="False"></asp:Label>
</td>
<td>
<asp:Label ID="lblP1" runat="server" Text="This is a test True/False Question."></asp:Label>
<asp:CustomValidator ID="cvTwo" runat="server" ErrorMessage="Please select correct answer true/false." ToolTip="Please select correct answer true/false." ForeColor="Red" ValidationGroup="Employee" ClientValidationFunction="ClientValidation1">*</asp:CustomValidator>
</td>
</tr>