I believe that proftpd - by default - does not follow symlinks. You'll have to give an 'Allow Symlinks' directive in /etc/proftpd.conf (obviously by editing the template). Read the docs on the proftpd site. Before you do this, though, you might want to make sure the permissions on the symlink are correct. If you're connecting through samba & ftp as the same user, this shouldn't be an issue, but if you're using the a different user for ftp, make sure the r and x bits are set for global access.