AutoPostBack On CheckBox Control Sometimes Fails

If have the below markup.

<asp:checkbox id="chkTVLic" runat="server" text="TV Licence" oncheckedchanged="chkDocs_CheckChanged"
autopostback="true" CausesValidation="false" />
<asp:panel id="pnlTVLic" runat="server" visible="false">
<div class="toggle-item-link1 document-date">
<asp:panel id="pnlTVLicIssueDate" runat="server">
<p>
Please enter the date of issue
</p>
<div class="fm-req">
<asp:textbox id="txtTVLicIssueDate" cssclass="tb size2" runat="server" onblur="return true;"></asp:textbox>
<cc2:calendarextender id="caleTVLicIssueDate" runat="server" targetcontrolid="txtTVLicIssueDate"
popupbuttonid="ibnTVLicIssueDate" popupposition="BottomLeft" animated="true"
format="dd/MM/yyyy">
</cc2:calendarextender>
<asp:imagebutton id="ibnTVLicIssueDate" runat="server" imageurl="../images/img-calendar-day.png"
alternatetext="Calendar" tooltip="Pick Date" cssclass="date-picker" />
<asp:requiredfieldvalidator id="rfvTVLicIssueDate" CssClass="error" runat="server" controltovalidate="txtTVLicIssueDate"
display="Dynamic" errormessage="Required" setfocusonerror="true" validationgroup="TVLic"></asp:requiredfieldvalidator>
<asp:comparevalidator id="cmvTVLicIssueDate" CssClass="error" runat="server" errormessage="Not a valid date"
controltovalidate="txtTVLicIssueDate" operator="DataTypeCheck" type="Date" setfocusonerror="true"
validationgroup="TVLic" display="Dynamic" cultureinvariantvalues="true"></asp:comparevalidator>
<asp:customvalidator id="cuvTVLicIssueDate12Months" CssClass="error" runat="server" controltovalidate="txtTVLicIssueDate"
validationgroup="TVLic" display="Dynamic" onservervalidate="cuvDocIssueDate12Months_ServerValidate"
errormessage="Document must be less than 12 months old."></asp:customvalidator>
</div>
</asp:panel>
<asp:panel id="pnlTVLicApprove" runat="server">
<asp:LinkButton id="lbnTVLicApprove" runat="server" CssClass="screen-hide"
alternatetext="Confirm TV Licence" tooltip="Confirm TV Licence" Text="OK" CausesValidation="false" OnClick="lbnApproveConfirm_Click" />
<asp:imagebutton id="ibnTVLicApprove" runat="server" imageurl="../images/img-accept-doc-off.png"
alternatetext="Approve" tooltip="Approve" cssclass="approval-btn" causesvalidation="true" validationgroup="TVLic" OnMouseDown="HandleApproveClick('TVLic','lbnTVLicApprove');return false;" OnClientClick="HandleApproveClick('TVLic','lbnTVLicApprove');return false;" />
<span class="approval-label">Accept document:</span></asp:panel>
</div>
</asp:panel>


The app is written in c# but i havn't posted any actual code as all the user code related to this markup seems to work fine.

The problem is the CheckBox chkTVLic has causes validation set to false and autopostback set to true. So whatever happens when i check and uncheck the checkbox it should postback. Most of the time this is exactly what it does and the result is to show and hide pnlTVLic when it is checked and unchecked. However if any on the validators within the panel fire, the checkbox does not cause a postback the first time. It will on all subsequent times but never the first. However it should ALWAYS cause a postback. What could be stopping it. Before someone asks there is no use written client side code, everything is pure .net markup and c# code.