To get updated content from the feeds you need to run the following scripts (in this order) on a daily base
# crontab -e
0 1 * * * /usr/sbin/greenbone-nvt-sync > /dev/null 0 2 * * * /usr/sbin/greenbone-scapdata-sync > /dev/null 0 3 * * * /usr/sbin/greenbone-certdata-sync > /dev/null
If there is any issue during the sync the scripts should give you additional info.