Builds with Scheduled Build Trigger Never Fire

Using TeamCity 2017.1.1 (build 46654), I have a few nightly builds that stopped working. I used to the advanced cron feature, forced days 1-7 and the same time at night in a template as borrowed from other engineers, copied setting by setting (those former, not mine continue to work without a hitch). I tried using the basic scheduled trigger, daily, at the same time. Still they go ignored.  I have tried disabling and re-enabling it in the template. I checked the derived builds from the template; all are properly enabled. I tried changing times hoping some state server-side on TC is stuck. But nothing changed, and still these nightly builds never fire at the same time, and never log an error.

It worked consistently for one, but failed on the others based on the same template. I checked the audit history and it is the only parameter that is the shell script invocation, nothing to do with any other XML element beyond this parameter. There is no failed build at night, nothing in the queue when I check. I have not seen anything like this, nor has anyone on my team.

What can I do to troubleshoot missed scheduled triggers? Any ideas?