The reasoning behind this issue depends on the type of Facebook post:
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 aren't carried over to the original ticket, nor do 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 added to your posts after the ticket was closed won't be pulled into Zendesk through the integration. They still exist in the post on Facebook itself.
For different issues or questions, see these articles: