By default FileZilla is configured to transfer more than 5 files at a time, which means that each time when you make file transfer, the application will initiate more than 5 simultaneous connections to the server.
If you are hosted on a shared server and trying to transfer many files, the server firewall may block your IP address due to excessive number of simultaneous connections from one and the same address.
To reduce the number of simultaneous connections open Edit -> Settings -> Transfers there you will see a field called Maximum simultaneous transfers Change the value to a lower value and then click ok.
