Can't access Mac pro via File Share (icon appears in shared sidebar)

Hi all

Lately I've been having File Sharing problem. I have an iMac, Mac Pro (2010) and a Macbook (white), now the moment that the iMac and Macbook are open I see my Mac Pro in the "Shared" sidebar, so everything looks good "they" see it BUT the moment that I try either one to connect to it, it say's "Connection failure". Now when I try to connect to my iMac and Macbook from my Mac Pro no problem at all and connects without having any problems and see both iMac and MacBook in the sidebar like the others. So why can't "they" connect to my Mac Pro (they used to not so long ago) even though they appear in the sidebar?? I have turned off/on File sharing in my Mac Pro but nothing, rebooted Mac Pro nothing, turned off/on WiFi nothing, checked Firewall and it's green so what's keeping my iMac and MacBook not to connect??? I tried just connecting to the internet with just WiFi and unplugged ethernet to make sure that my WiFi gets and receives signals and it does so the card looks to be working as it should.

Thanks for any help with this!!!

Serge