netboot problem

Hi All I'm hoping someone can help with a netboot related issue. i'm trying to deploy an osx 10.4.9 image to a number of imac G5's. the netboot service is running on OSX server 10.4.9 on an xserve G5. All services are running as expected (afp, nfs, OD). Both a netinstall and netboot image were created from .dmg file. system image utility carried out image creation without error and the netboot service is up and runing. On the client side. the globe icon appears for about 20 seconds, then the spinning globe w/ apple icon appears for a few seconds before the spinning progress indicator appears and then nothing happens - it doesn't advance beyond this stage. I've done the basic checks - the target machines can get ip's from our dhcp server. When logged in locally to the clients the network images are available as options in the startup disk pane. The netboot server logs don't appear to indicate anything unusual (the clients and server exchange bootpd (bpsd) communication). What is strange is that when the clients are set to boot from the network image the system doesn't allow me to enter verbose mode on startup - so i'm unable to see what problems the client is having. I'm guessing this is unusual. Any ideas ?