Are you sure you are using the right port? Most FTP clients (and daemons) default to 21 (eg. ftp 10.21.100.1 21), but that's not always the case (it's worth using the port everytime just incase). In everything I've ever done with FTP (mainly IBM Unix boxes connecting to PC's or the other way around) it's normally a problem with access rights (user hasn't the right to view or upload/download etc) for the user or an incorrect port number. But if it works from the command prompt then pretty much every other FTP client should work with the same configuration. Also sometimes only certain IP addresses are accepted by the FTP Daemon, so is it the same PC you tried all the FTP clients (including the successfull one)? Oh, and if it's Vista perhaps worth running the clients in admin mode (annoying Vista...)
Cheers
I am 99% probably lying in bed right now... so don't blame me for crappy typing
Current fave quote : "She was like a candle in the wind.... unreliable...."