By default FileZilla is configured to transfer 5+ files at a time, which means that each time when you make ? transfer, the application will initiate 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.

There is an easy way to reduce the number of simultaneous connections/transfers in FileZilla. In order to do so open the application -> Edit -> Settings -> Transfers. There you will see a field called Maximum simultaneous transfers - change the value to 1 and click on OK to save the changes.