User61956409 posted
Hi JagjitSingh,
Please use F12 developer tools to check if your asp:CheckBox control CheckBox1 is rendered like below.

Besides, as yogyogi mentioned, to troubleshoot the issue, you can try to create a new web page and test same code of asp:CheckBox control CheckBox1 and check if it works as expected. And please check if using event.preventDefault() method
(or other script) stops the default action.
With Regards,
Fei Han