Make iptables like your ftp sessions
If you manage your firewall by hand have issues with connection trough ftp, try adding this.
# FTP Helper (beginning of script)
modprobe ip_conntrack_ftp
modprobe ip_nat_ftp ports=21
# ...
# other rules
# ...
# FTP Helper
iptables -A OUTPUT -o eth0 -p tcp --sport ftp -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport ftp-data -j ACCEPT
# Drop rule
#(end of script)