Let me brief you some explanation why it not work for your existing articles :
1. Those Joomla article created it before install Easysocial on your site.
2. It will not auto generate stream item for your existing Joomla article when you install Easysocial on your site.
3. In order to automatically generate these article stream item, either you create article from backend or frontend, it will generate it because both also go to the same function to store the article content in database.
4. But now the issue is those article already created it before you install Easysocial, so that is no way to re-generate again for this 'create' article context type stream item on Easysocial.
5. When you re-save the article from backend, it will only generate 'update' article context type stream item on Easysocial, so this 'update' article context type will not associate your article page comment to this 'update' article stream item.
The only way I can think of is manual work convert 'update' to 'create' context type from database.
You can give it a try this following instruction how to manual done this : (Remember do the backup first before you do this)
1. Enable this 2 setting from article app (backend > Easysocial > application > search 'article' >
https://take.ms/uKB8x).
2. Edit one of your existing article from backend which got some comment added, then click save/update.
3. Once save already, open your PHPMYADMIN > select your current database > click on the SQL (open 2 browser tab for this same page) then execute these following each SQL to each browser tab.
Remember replace #__ to your current table prefix
select * from `#__social_stream` where `context_type` = 'article' order by id desc limit 10;
select * from `#__social_stream_item` where `context_type` = 'article' order by id desc limit 10;
You will probably see this similar data.
https://take.ms/Q20W1a
https://take.ms/kaC4J
4. Edit that 2 table line of row data, replace context_type value from 'update' to 'create';
Final result is :
https://take.ms/zRCSY
5. Once you updated this, you can check your site Easysocial user dashboard page, it should show a create item for this and associated with the comment as what user comment on that article page.