Reading back through prior emails. . . TFTP client requests packets *are* making it to the TFTP server. So it seems like something on the TFTP server itself.
Like previously mentioned server side firewall/iptables/tcp-wrappers/selinux are all possible culprits.
Hmmm just thought of something else, what are the file permissions of the file you are requesting? Try `chmod a+r filename`?