Unable to delete files via FTP?

I have a couple of FTP users who can access my (pseudo) server running vsftpd under Mint Linux. They are restricted to a specific directory and below via chroot(). Whilst they can access the files without problem they are unable to delete them when they are no longer required. The permissions on their base directory, and all sub-directories is 'drwxrwxrwx' and all files '_rw_rw_rw_'. I have searched the web but been unable to find an explanation.