Indy 9.0 TidFTPServer Binding Error

I am trying to write a ftp server with Indy's TidFTPServer.
I am able to download just fine with one user connected.
If However, I have multiple users connected and they try to download while
another download is in progress, My app blows up with the following
message: 'Could not bind socket. Addresses and port are already in use.'

I must be missing something.

Thanks for any help