Scheduled lock is late or doesn't start
On Android, when your phone sits unused for a while, it goes into a deep sleep called Doze. That can delay a scheduled lock.
If a lock didn't start on time, wake the phone and it should start.
To stop it happening again, make sure the Alarms permission is on. Then open the Settings tab and find Background restrictions, under Schedules. If it says battery saving is on, tap Fix.
Your phone can wait. The rest of your day can't.
Free on Google Play. Premium is optional.