While controlling today (ATL CTR), many bordering sectors were online. CLE and JAX CTR were both on, and I could see them. BHM and ATL APP were both on below me, and I couldn't see them. When I handed off an aircraft using the common sector ID, it worked fine, and they received the aircraft. Why do they not appear in my controller list? I have changed nothing at all in my CTRL + F menu, nor in my CTRL + S menu. The same thing occurred when I controlled IND CTR this morning, I had the same problems only seeing about 1/2 of the controllers that were actually logged on.