Unattended remote control problem

[BudVitoff]

I have an iMac, and I work with an identical iMac, a Macbook Pro (same software), and a Dell PC.
All four of us keep powered up continuously, and TV is loaded at login.
I have no problem connecting to any of the others unattended, but none of them can connect to me unattended.  It sounds like I have a wrong setting somewhere, but I have no idea where it is.

Can someone straighten me out?