My AT&T Nexus 6 used to get the monthly security updates on the month they were released. They'd arrive maybe two to three weeks after the date on the patches.
I realized the other day that I hadn't seen an update in some time.
I'm still at January 5, 2017. There was that nasty Broadcom vulnerability, and I'm not patched for it.
The update check says I'm up to date.
Google support documentation says I should have a few more months of security updates, even if new Android versions aren't guaranteed.
Any suggestions to get the monthly security patches going again?