Why can't I @ mention in Discussions?
In a Discussions thread, you can @ mention anyone who's in the thread and/or anyone of the last 500 people you have followed. The limit was set up for a technical reason. If you can't @ mention someone, check to see if you are following them. If not, try following them, and you should be able to @ mention them. If you are following them already, try unfollowing them, and then follow them again. You'll then be able to @ mention them. We're working on making this easier in the future.