Can't see Meta boxes

A theme I’m using came with several Meta Box integrations/developer modifications (things like setting a subtitle, changing post layouts, etc). I’ve since bought a developer license to add some more things of my own and migrate away from ACF.

All the developer meta boxes show correctly on posts, but whenever I create one it doesn’t show in the post editor screen – if I go to the Screen Options setting at the top of the page and toggle on/off whichever metabox I created, it just shows a blank space of where it should be, but no box/custom fields.

I tried with the Builder extension installed, and with the builder function enabled via the AIO plugin, but neither worked.