This means that not all ZFS pools might be imported after the system has completed boot, even if the underlying devices are present and functional. A quick and dirty solution would be to run zpool import <poolname> after boot, either manually or from cron.
A better, more elegant solution is instead to hook into systemd events and trigger zpool import as soon as the devices are created.
First you will need to figure out the systemd disk ids that you wan systemd to notify you about.
This can be done by running systemctl list-units --all --full | grep disk from a shell with all disks mounted and/or connected. Disk ids are in the first column and the last part of the disk id is the same as the name found under /dev/disk/by-id/.
iSCSI disks helpfully report iSCSI_Storage under the last column.
Once you have figured out the systemd device id you need to create a custom service unit like the following (/etc/systemd/system/itank-pool.service):
Now try rebooting your server or systemctl reload, export the pool and restart the iscsi service. You should notice that just after the iSCSI devices appear systemd will run the zpool import service (one shot).