If you are using webMethods built-in FTP services (pub.client:ftp),ftp folder these behaves same like regular FTP process.
If you are using TradingNetworks Delivery Method like selecting (primary FTP or secondary FTP)protocols this can be configured such a way to retry the process until the FTP TASK is success.(GD).
Basically the webMethods FTP process can be made asynchronous/synchronous mode.