Can't Respond To Whispers In Game

Lately I haven't been able to respond to some of my friends who message me while I'm in a game. Whenever I do /r or /msg [Friend's Name] I get a message stating that they can't be found on my friends list. Is anyone else having that problem? I dug around and tried to find a solution but the latest posts I found about a problem like this was in 2013 and no reds responded to it. :-)