Passive FTP is not enabled with pure-ftp by default. To enable this, edit /etc/pure-ftpd.conf and uncomment out this line:
PassivePortRange 30000 50000
This means that FTP will answer passively on the range of ports between 30000 and 50000, so you may want to narrow the scope to something like 30000 to 35000. Once you do this, you need to restart FTP and open the range of ports you selected in your server’s firewall.
If you’re running ProFTP, you can edit /etc/proftpd.conf and add this line, if it doesn’t already exist elsewhere:
PassivePorts 30000 50000