Buffer overflow problems

Hi all

been testing out local root exploits on my old fedora 4 box, i was trying for a while to exploit the code, got it once and it worked fine, wrote a new line to /etc/passwd giving me a new root account with no password. Unfortunately I came to try it again and no matter what I seem to do, I always get the error "cannot write to /etc/passwd", even what I switch to root and run the same command.

The program being exploited has suid bit on, but is owned by the unprivaleged user. This worked once before but I can't recreate it to save my life! really annoying me! any ideas? I know its vague I should really post the code here but its on my other laptop.

Any help appreciated.