Install Mac OS X via network across different subnets
I'm late to this party but @klanomath's comments are correct- you probably need to open the network ports. If you can see the images on the local network but not the remote network then it's likely the traffic isn't making it through.

Watch the server's system.log file for messages like this from bootpd:

BSDP INFORM [en0] 1,0:1b:63:39:d0:9f NetBoot024 arch=i386 sysid=iMac5,2


Or get a packet trace from the server:

sudo tcpdump -i en0 -s 0 -B 524288 -w ~/Desktop/NetBoot.pcap


I usually open them in Wireshark and filter for "bootp or tftp" to see how far things get. If there isn't any bootp traffic from that client then something on the network is probably blocking it.