FTP problems

Hi
I’m on a network which has a firewall and I use WS FTP LE to upload and download files to and from a remote web server, however when I try to upload files, I often get a message saying “blocking call cancelled” or a timeout after uploading the first file. It is as if it allows the file to be uploaded but stalls when waiting for a request from the server. It seems to send up files when it feels like it.
N.B. it used to work with the firewall before and it just stopped working for no apparent reason, so I don’t think it is the firewall.