'Next page' tag not working

Hi, I’m using a child theme and the next page tag isn’t being recognised although I do see that the content-single php file already supports it.

I’m using the full-content layout and the read-more tag is recognised. Do you have any ideas why the next-page tag wouldn’t work?

Update: this doesn’t seem to be a theme problem. I’ve just found the same problem when I applied the 2014 theme so it must be a deeper problem.