Use the Samba Web Administration Tool. It rocks.
It should help you finger out the problem. There is extensive HELP...
But, the surest thing I can tell you, if the directory is not owned or made writable to the username you are using through through UNIX... well that'd be yur prob.
If the Directory is owned by root and grouped root, has perm -rwxrwx---, you'll never write to it. You have to rememebr SAMBA is ON TOP of UNIX security, SAMBA follows the rules just like any other application.