I just had a really hard time tracking down a problem in which some of my scheduled tasks (those with "*/<n>" in the minute field) were being sent, but others (those with "<n>" in the minute field) were not. The problem finally went away when I upgraded cron-descriptor from v1.4.5 to v2.0.6.
Looking in requirements/default.txt for Django Celery Beat v2.8.0, I see cron-descriptor>=1.2.32, but that is clearly not accurate.
Edit ---
Initially, upgrading cron-descriptor seemed to have fixed the problem, but now the problem is back, so the cause is actually a mystery. Is anyone else having a problem like this.