OnCheckChanged not fired on CheckBox uncheck

I know this question may have been asked already, but none of the answers work so I'm asking to see if anyone has any better ideas. Very simply, I have an asp:CheckBox as such:

<asp:CheckBox runat="server" ID="IsPrimarySigning" Font-Size="Large" CssClass="signer-margin" OnCheckedChanged="SignRemotely_Checked" AutoPostBack="true"/>

And the OnCheckedChange handler method:

protected void SignRemotely_Checked(object sender, EventArgs e) {}

The code inside it is irrelevant because I put a breakpoint on the first open curly brace and it's never hit by unchecking IsPrimarySigning. However, it works and the breakpoint is hit when I check IsPrimarySigning. Any ideas?