Comment box showing on some but not all posts?

Yikes, that page almost used up all my bandwidth just to look at your problem. You should compress your images or put fewer on a page (you can use paginated posts).

For the comments, the comments section is in the HTML source of the page, but it is empty. If you are not using a plugin to modify comments, then your theme is just not doing it right since you said you made sure comments were enabled on that post.

The Settings > Discussion options apply to posts as they are written, so if you change them, it only affects whatever you create after the change. If you had comments disabled for a time, any posts written then have them disabled and changing the global setting does not affect those posts; it would only affect new posts. But the Discussion settings inside each post override the global setting.
The theme should show the comment reply form for any page that has comments enabled, and should show any existing comments even if comments are closed.
The core comment template handles all this correctly, but some themes don’t use the core template, and mess it up.

Sorry Joy, I use smush to compress image size when I upload them — do you recommend something else?

Do you know why it would work on some posts rather than others? Should I contact the maker of the theme? I have never had comments disabled so all of my posts should have had a comment box from when I published them

Please let me know how to fix this. Thanks!

The theme support forum is the place you should be…

Thanks Joy. Turns out it was not a theme issue, but just a simple settings fix. For anyone else with this issue:

The comments will show up depending on your WordPress comments settings. Try going to Settings > Discussion and see if you have checked the option called “Automatically close comments on articles older than XX days