This is but a coincidence, they happened to be afk, checking unban apps or watching Pewdiepie. Also, as any other players they might have a mod that plays a sound when you write their username in the chat, try that (not the nickname). They are not constantly in game, u have to understand that. I myself have to ask/send a message over 10 times sometimes because they are afk, however! Instead of doing that send them a mail, u cant message them if they are in vanish mode but you can do /mail send [name] that helps out.