Back to My Mac isn't working on my iMac.

Back to My Mac isn't working on my iMac. It IS working on my MacBook on the same network. The message is "ICloud sever not responding". Followed directions to resolve, but still does not work. Why, I wonder, not? I've read a little about TCP/UCP ports etc. but with application based firewall, there's no way for me to look at any specific ports. The help pages just keep sending me around in circles, over and over. I'm getting dizzy. Can anybody help?

Thanks!!

Johnfromgilroy