Can you check the debug log inside of Plex media server and filter for “trakt” to see if it’s attempting to send Trakt any data, and the status codes?
That might help us find what’s causing that issue.
Or, if you prefer, like the user above said, you can send an e-mail to support@trakt.tv and they can enable Plex debugging on your account to see what Plex webhooks are being sent to us.