Did you go into the post editor, scroll down to the Discussion section, and check the box that’s labeled Allow comments? If you do not see a Discussion section, click the Screen Options tab in the upper right corner of the screen and check the box labeled Discussion.
If you want comments to be always available on your posts, go to Settings → Discussion and uncheck the box labeled Automatically close comments on articles older than ____ days. This will only affect posts which are currently open, however. You’ll have to go and re-activate comments for all of the posts which have them closed. You can do it fairly easily by going to Posts → All Posts and setting Allow Comments for all of the posts on a page at the same time.
