You configured a weekly full scan to run each Monday i.e at 10:00 AM, with a retry interval of 3 days if the original scan is missed.
You notice few machines misses a scan and pushes to a next week.
If a machine missed a scan for example the machine was off Monday or the machine got started at 10:30 AM so post the scan schedule. the retry scan with SES has a defect
where a Registry everytime when SMC/machine reboots is set to time(null). Resulting in Schedule time to shift to future than now and miss the scan even if it is in time window.
It is identified as a known issue and a fix will be available in upcoming release.