Why are my comments closed, even though they are enabled on the post?

There are 2 places where comment settings live

1. The Settings->discussion menu (this is probably where your issue is)
2. The individual post

Example 1
A common setting that prevents comments from working is "Automatically close comments on posts older than..."
This will override the comment setting on the post. You can uncheck this value, or increase the duration to enable comments. 


Example 2
Enabling comments on the individual post

So, when you run into issues with comments, first see the Settings->Discussion tab, and then double-check that the individual post is set to allow comments.