Question

$('#comment-form-subscribe input:checkbox:not(:checked)').prop("checked", true);

If checkboxes are unchecked, manually check them, but this doesn’t seem to be working.

Answer 1

You might have a selector issue: I’d check that the length of the jQuery object returned isn’t 0. This is what I typically use:

EDIT: as @nbrooks pointed out, :input is deprecated. This might work:

$('#comment-form-subscribe').find('[type="checkbox"]').not(':checked').prop("checked", true);