If you apt-get install proftpd on Debian it seems that support for resuming an FTP transfer is off by default.
For some reason the setting that controls this isn’t exposed in the default proftpd.conf, but basically it’s just a case of adding the AllowStoreRestart option to your proftpd.conf (ie, ‘AllowStoreRestart on’), then restarting proftpd via /etc/init.d/proftpd restart, and you’re away.