1. Depending on your network. A lot of time, multicast will cause network problem, especially for a corporate network. You have to know how the network been deployed.(how the switches configured);
2. Most of the time, check you client boot disk. If you have any error message booting from the network boot floppy, you will fail.
3. Manually set hard IP on client boot disk;
4. Try Unicast rather than multicast;
5. If the client computer is not in distance to the server, use cross-over cable.