Note. The version of ProFTPD that ships with Plesk does not allow “resume” for security reasons. This is a good thing. Resume woudl allow a writable fiel to be corrupted as opposed to you having to replace the file… or so they say.
Anywa, here is the voodoo to turn it on (remember to turn it off after)… thanks Tech Support Guy!
add to /etc/proftpd.conf
# allow resume in downloading
# use this one at your own risk, some ftp deamon will
# not start with the download resume in
# HideNoAccess on
# AllowRetrieveRestart on
# allow resume in uploading