One of my image sizes isn’t showing up in srcset

Just gong to post the answer here too. @Dan linked to another question that had the answer.

Here’s some documentation on the new responsive images feature:

Responsive Images in WordPress 4.4

The default max-size that will go into a srcset is 1600px wide. So if you plan on using images that are bigger than that, you need to change that.

It can be changed like this:

add_filter('max_srcset_image_width', function($max_srcset_image_width, $size_array){
    return 2000;
}, 10, 2);

Where 2000 is the new max width.