Are you on Windows 8? I had the same problem, the solution was to turn off firewall or this (its a copy paste from a note I had, cannot find the bookmark…):
**Until we explored the issue in detail, the most common Swarm problem was blocked communication between different Swarm agents. Firewalls are the culprits. Windows Firewall will block Swarm by default, even if the user chooses to allow Swarm to do outbound communications. Because Swarm communicates in a peer-to-peer fashion, it must also be configured to allow inbound communication from other agents.
Programmatically, the Windows Firewall can be configured to permit Swarm usage with the following commands. We also do IP filtering to make sure these firewall exceptions only happen on our local network — replace x.y with your actual leading IP octets.
Allow Swarm ICMP pings: netsh advfirewall firewall add rule name=”Unreal Swarm” dir=in action=allow enable=yes remoteip=x.y.0.0-x.y.255.255 protocol=icmpv4 interfacetype=lan
Allow Swarm TCP commands: netsh advfirewall firewall add rule name=”Unreal Swarm” dir=in action=allow enable=yes remoteip=x.y.0.0-x.y.255.255 protocol=tcp interfacetype=lan localport=8008,8009,54430,56574,56587 remoteport=8008,8009,54430,56574,56587
Uninstall firewall exceptions: netsh advfirewall firewall delete rule name=”Unreal Swarm”**
I hope this helps you.