If the PutFile method finished without an exception, then it means it actually succeeded. Perhaps you uploaded the file into an unexpected directory?
To find out what is going on, please assign the LogWriter property before calling the PutFile method, and run the code again. Then check the log.
var client = new Sftp();
client.Connect(...);
client.Login(...);
client.LogWriter = new Rebex.FileLogWriter(@"\\Test\Log", Rebex.LogLevel.Debug);
client.PutFile(@sourcepath + filename, destinationPath + filename);