I set up a custom Smart Home Monitor to send me a notification when a door is left open for more than five minutes. I have it linked to a contact sensor and everything in the app looks to be correct, but it never fires. I want it to fire anytime, if the system is armed or not. I’ve looked at other apps, but I don’t understand why this isn’t firing.
It would help to see the code. however…
Load the app with debug statements and use the IDE Live Logging to see what is happening. Your problem may very well be related to the scheduling issues ST is suffering from these days. My “Remind to lock…” app sounds similar and it too fails more often than not 
It is the Smart Home Monitor app, so I don’t have the code for it. I this I found the issue. I didn’t have a mode set for it to run in. I assumed if I didn’t pick a specific one it would run all the time, but it didn’t. I set it to run in the home mode and it started working.