@taoeffect @ntnsndr originally the only way to notify anyone was with an inline mention -- by tagging their account. It's ugly, I hate it, a bunch of people rallied around this and got direct mentions working so you don't need to tag them to be able to notify them; it could be done in the UI outside the text content.

My only guess is that they're not including your tag in their reply and the client they're using (phone app, custom web interface, etc) does not know how to do direct mentions.

They're doing something weird.