Try this,
got to /Macintosh HD/Library/Preferences/ and look for a file by the name of com.apple.alf.plist (this is the firewalls prefernces list) even if you don't have the firewall on it can cause issues with file sharing and screen sharing alike (it was common when i was second line applecare) now just drag the file to the trash and empty the trash then restart the mac and it should be fine.
hope this helps