I've been on TeslaFi since August 2016. During that time, I never had a problem with my Model S failing to sleep (except when I changed the settings known to interfere with sleep). Beginning in August 2019 my car no longer was willing to sleep. Settings were fine, logs show TeslaFi giving the car a chance to sleep. Nothing helped and I was busy with other matters.
Fast forward to November, 2019 and my car gives the alert to have the 12 V battery service/replaced. Tesla replaced the battery yesterday and with no other actions on my part, the car is back to sleeping just fine. There wasn't even a software update to the car, so the only change was replacing the 12 V battery. (Aside to the other old timers - this is my first 12 V battery replacement, so I got about 4 1/2 years. Not bad given about all the concerns back in 2015 and earlier with many 12 V batteries failing after a year.)
Might be worth adding this to the knowledge base article on reasons why a car won't sleep. As the 12 V battery degrades and no longer holds a charge well, the need to more frequently recharge the 12 V battery seems to keep the car from going to sleep.
Caveat. My car has AP1 and the MCU1 control unit, so it remains to be determined whether cars with the newer control units behave the same way. In any case, it will be a few more years before the 12 V batteries degrade on those cars. For now, I expect this to be an issue mostly for older Model S.
Customer support service by UserEcho