Could be a couple of things. I think there's a limit to how many people or you can @ in one post. Could be something with him specifically. It works, though.
There are a few weird things with the system. Like not receiving duplicate alerts. So if you're already watching a thread, you won't receive a notification for both a new post and a mention. Things like that.
I'm trying to figure out what the max number of mentions is per post, but I can't put my finger on it.