Raft: can’t edit slug

Hi! I’ve created a page, which has taken the standard form of [url]/?page_id=78, but I can’t seem to edit the slug.

Usually what I’d do would be to go to Pages, hover over the page in question, click Quick Edit and then change the slug to the title of the page. I’ve done that, but it isn’t working – the correct name ([url]/writing) is in the slug, but then if I navigate to [url]/writing, I get a 404, whereas the page is still accessible at [url]/?page_id=78

I’ve never had this problem before so I assume it’s linked to the theme – what do I need to do to fix it?