If you select each file one at a time and download them via FTP, they all succeed.
If you select the same group of clips simultaneously and try to download them via FTP, one or more may succeed, but almost all the other downloads fail with a service not available error (or some other error).
How to rectify the problem:
The XDCAM decks only allow one MXF file access to occur at a time. The exception to this rule is the Professional Media Station (which is a multi-tasking machine).
You must adjust the transfer settings in the FTP client to only allow one transfer at a time.