Why aren't new comments from Facebook posts or ads being added to tickets?
Facebook standard wall posts
The Facebook integration only pulls in comments from standard wall posts for one week after the post's creation. Comments added after that point are not carried over to the original ticket, nor they create a new ticket in your account.
When Facebook ads are converted to tickets, the integration pulls in any comments made in the post up to a year after its initial creation. However, any comments that were added to your posts after the ticket was Closed are not going to be pulled into Zendesk via the integration. They still exist in the post on Facebook itself.
For more information, see the article: Setting up your Facebook channel.