tftp: ARP timeout

[khansari@linux.ce.sharif.ac.ir]

Dear Kuhlmann,
What is the netboot mailing list?I sent email to netboot@baghira.han.de but it was rejected!!
Anyway, I setup remote boot config for Linux diskless stations.None of them has hard disk but they have a static IP address in the server which will give them upon booting. The ImageKernel in located at /tftpboot/ImageKernel
The station are using bootpd and tftpd via inetd to get the necessary files to boot.
I used a 3COM network cards.The system were working fine but due to a unknown problem i got into problem:

The client reports me the correct configuration as of
local IP, server IP, and filename.

But then I get:
TFTP: ARP timeout
Insert boot disk and press a key

Any idea how to fix this?
What's wrong here?
I'm not a member of the list, Please reply to my email address too.

Thanks,
Mohammad