I seem to be having the same problem as everyone else.
According to “Platform Status”, time based routines are “Degraded Performance”. http://status.smartthings.com/ What ever that means.
At first I thought it was related to the fact that I had managed to get two Security Monitors attached to my account after I added a Cree Connected bulb. ST support did get the extra one removed but they were slow about. Took over a week, so I think they are swamped with service requests.
Funny thing is that the smartapp Rule Machine Rule machine - as per the app developer, this app is no longer available for new installs, distribution, or support has similar functionality to the built in apps and has yet to miss a time based trigger, although I have only used it for a few days.
In addition to missed time based triggers, I am also seeing a lot of “unexpected errors” when looking at the notification messages and activity feed, and both lists are extreamly short. E.G., even though my week end events are very similar to the week day, the only event messages that were shown were from manually triggered events. Months ago I would have dozens and dozens.