It sometimes takes a lot of digging to work through network connectivity problems, but in general if the clients can't find the server then it's almost always true that:
a) the switches and/or routers in the network aren't configured for IP multicast support
b) if the clients don't get through from a different subnet, then the WINS fallback for multicast being unavailable isn't working either.
Unfortunately, explaining what's involved in a) depends somewhat on the network in question.
The usual problem is that Layer3 switches typically have IGMP snooping on by default, but have IGMP query generation off by default; this means that multicast groups only live for a short time before being timed out by the switches and then no traffic gets through any more. In this case, either the switches or a router need to be configured to generate the IGMP queries so that multicast works properly.
If the client and server are on different subnets, then it's usually just a matter of turning on a "understand multicast" setting in the router, and it will then pass multicast traffic between subnets and also generate the proper IGMP queries to the networks they serve as well (keeping the switches happy).
To diagnose this I generally ask people to take one Wireshark capture on a client that isn't connecting (run it over a couple of minutes) and one Wireshark capture on the server (lasting a couple of minutes, not running any tasks at the time). That usually lets me see whether the server and/or client machines are being sent IGMP queries and whether the multicast traffic is getting through at all or not, and also whether subnetting is involved.