ISSUE: Comments are showing on some posts, but not others
WordPress has a setting that will automatically close comments on posts older than a certain number of days. If your newer posts have comments enabled and your older posts do not, it’s likely that this setting is turned on. Go to Settings > Discussion to turn this setting off. Look for the setting labeled "Automatically close comments on posts older than ___ days." Uncheck the box, and save the settings.
Here is a video showing you this setting: