Some checkboxes work and other don't- why?

In my Django project, I have one page that displays a list of items and a checkbox next to each item. For some reason, some of the checkboxes are not 'checkable'. I can't tell why this behavior changes because most of the checkboxes work fine. Why would some checkboxes work but not others, when I'm displaying them all the same way?

snippet from mypage.html:

<form method="get" action="next_step">
    {% for item in myitems %}
        <input type="checkbox" name="yourselection" value="{{item.id}}"> Item info goes here
    {% endfor %}
</form>