problem with ftp access

hi, everyone, I have a small problem and I don't understand it, maybe because i'm missing something quite big... but when I read some faqs, nothing really matches I have several sites and ftp on a suse 7.2 box running proftpd. I can ftp into a ftp "mysiteA" without any problem while on another site if i ftp "myftpB" , I Can log but have the message 500 illegal port command 425 can't build data connection : connection refused Why this on one site and not on another one ?... what can I do to fix this ?... Regards stephane