Don,

My guess is that the NFS bit isn't working. I don't know what NFS server you're using, but on my linux box, I add the following line to /etc/exports for each receiver:

/tftpboot/x.x.x.x *(rw,insecure,all_squash)

where x.x.x.x is its ip address.

What os are you running?

Cheers
Paul