I have a CheckBox on an ASP.NET Content Form like so:
Additionally: Check for any errors in the JavaScript console.
I experienced the same exact issue described by OP except that it only happened in Safari (checkbox worked fine in Chrome and Firefox). Upon inspecting the JavaScript console, I found an error that was being thrown by a malformed jQuery selector.
In my case, I had $('a[id*=lbView') which was missing a closing ]. This threw an error in Safari but, surprisingly, not in Chrome nor in Firefox.