It seems that the checked property of a checkbox cannot be modified, when the event is from the checkbox itself.
Here is small example:
<input type="checkbox" onclick="this.checked=true; return false">The checkbox is not checked after click. Why is this.checked=true ignored?
It's happening because you're returning false. This value is assigned as the input value. Try this way:
<input type="checkbox" onclick="this.checked=true">