the '530 No transfer-slave(s) available' message is sent because the 'master' can not find a suitable 'slave' to initiate the file transfer from. This can be caused by the slave with that file being offline, an incorrectly configured slave or lack of configured slave(s) altogether.
I posted this same explanation (with example of working drftpd setup) here:
http://www.inicom.net/forum/showthread.php?t=15274