I think this is a bug with the plugin.
Actually, that’s unlikely. Checkboxes have been part of the plugin for quite some time and adding fields is a fairly robust process that hasn’t changed too much.
The first thing to take a look at is what you’re doing when you’re adding the field. Checkboxes require a “checked value” (which is the field’s value that is saved when the box is checked). If you leave that empty when you add the field, the field is not actually added because you did not complete all of the required settings. There is an error message that indicates this, but I know that get’s missed by many people because I often see installs that have quite a few messages displaying at the top of the admin already so they just don’t see the error messages when they are displayed.
Second, make sure that the field is set to “display”. I’m sure you’re doing this as you obviously are able to successfully add a text field. But it is something to point out.
Make sure that if you’re using a cache plugin that you clear the cache when adding a field. Similar to the text field above, I don’t think that’s the case here, but it should be checked.
Make sure you don’t have any CSS overriding the display of the field. I’ve seen this occur where a stylesheet from a theme or plugin has a general definition for something so it ends up overriding a bunch of other stuff. A property of “display:none;” assigned to the input would be something you’d need to look for. Use a browser inspector to see if the field HTML is in the form and if so, look at any applied CSS.
I suspect the very first thing I mentioned is the culprit. Make sure when you create the field that there is a value for “Stored value if checked”.
