Indy 9 ftp Put

I'm using idftp client to put files in a server. It works ok in many
customers, but some of them are saying to me that they got always a "read
timeout" at end of the transfers. The file is received ok in server side,
but i think that last ack from server is never received in client side. It
only happens with big file sizes. I 've tried Serv-U and Filezilla server
with same results.

They say that they can transfer files without problem with other ftp
clients.

Any ideas?

Thank you.