Custom Fields not showing in post edit or under screen options.

HI all, have trawled forum but can’t find this.

Only on 1 of my sites can I not edit the ‘custom fields’ on each post, such as ratings etc. The custom fields drop down should appear below the ‘excerpt’ dropdown. When I go to the top right and select screen options, ‘custom fields is not an option.

I have Switched off plugins and nothing.

Any ideas how to get it back? cheers.