If you are doing the FTP from the iSeries, you can use source members to store the input and output of the FTP.
OVRDBF FILE(INPUT) TOFILE(QTEMP/FTPCMD)
OVRDBF FILE(OUTPUT) TOFILE(QTEMP/FTPLOG)
FTPCMD and FTPLOG were created with CRTSRCPF QTEMP/FTPxxx
Once you do the FTP command, you can now go in and read the FTPLOG file to determine exactly what happended in your process.