There is the rare (the cause is yet unidentified) issue of sometimes not all WPPizza categories you have set showing up when going to WPPizza - Categories in the admin side of things even though they are happily showing up in the frontend.
If you have enabled debug, you might also see something like:
PHP Notice: Undefined offset: [abc] in [plugin-path]/wppizza/classes/class.wppizza.categories.php on line [xyz]
in your debug.log

To fix this, do the following:

• add a new – dummy – category
• drag drop that new category above/below another category
• delete that dummy category again
• reload the page

it should all now show up again as it should and any new php notices to that effect should disappear