here are some things that may cause the problem you describe:

1. Not using a wired connection for all stations/nodes
(e.g. connecting a laptop to the network via WLAN)
2. having the same IP-range on several network-adapters on one station (e.g. both WLAN and LAN starts with 192.168 etc)
3. Windows Firewall is enabled and not properly configured
4. 3rd party Firewall is enabled and not properly configured
5. Using switch or router that blocks or redirects multicast traffic