PRev/Next buttons missing

I set up a new wordpress site in the past few days. In doing so I noticed that my old WordPress site doesn’t display previous/next tags at the bottom of the pages like the new site does.

I don’t know what made them go away, and I’d like them back (blog navigation is hard without it!). I reviewed the style.css file (that’s part of my custom theme, minor modifications to twenty sixteen) and I don’t see anything that would change what I think are the CSS for .post-navigation, and I don’t remember ever trying to get them to go away.

An example page would be:
https://www.irelandbybicycle.com/index.php/2015/06/23/

There’s definitely blog posts before and after that.

(Now that I think about it, I’d rather have those tags at the top of the page, but that’s a different question once they’re back)