Make sure that the comments on the post are meeting the criteria you have set on the plugin’s Commenting options tab. If it seems that the posts comments should be getting pulled to WordPress, you could debug the issue by enabling the ‘Custom Fields’ screen option for posts on your site:
When that has been done, you will see a list of post metadata at the bottom of the edit screen. Make sure that the discourse_permalink and discourse_topic_id fields match the associated Discourse topic. If these fields are not present, it is possible that the post has been unlinked from Discourse. You can re-link an unlinked post by selecting the ‘Link to Existing Topic’ radio input from the Discourse meta box.