One possible reason I found now:
From time to time execution of the timer is delayed for more than one minute. When a cron task has to be executed in this minute, it is skipped.
I proved this by adding some code to cron's index.js (see attached file), which checks this and writes a console message. And indeed I found this message in log file:
Code: Select all
[2019-02-28 20:23:15.092] [I] [core] Cron: tDelta=2 minutes, time reset to last minute +1
I don't know the reason for these delays. My Raspi3 is only lightly loaded. But it happens. A Raspberry Pi is not a real-time process computer.