If I look at the history for a scheduled scrub it shows the following as the latest entry
62 February 1st 2016, 3:00:04 am December 1st 2015, 3:00:04 am running
However if I check the CLI
btrfs scrub status /mnt2/Proxmox/
scrub status for 48e9fe04-9f49-4c33-b9d7-12aefe745674
scrub started at Tue Dec 1 03:00:04 2015, interrupted after 10:15:05, not running
total bytes scrubbed: 602.21GiB with 0 errors
I suspect either the VM or the hypervisor got rebooted whilst it was running, however because of this subsequent scrub tasks have never run.
Ideally the rockstor service needs to keep better tabs on the status of the Scrubs and do something about about it should one not be progressing for whatever reason even if something is to simply alert the user to a potential problem.