"arrive home" event doesn't trigger with Apple Home setup.

I have both "leave home" and "arrive home" scenes with defined actions. I live alone, so there's only my iPhone to trigger when I leave home. When I do leave, that scene triggers and the actions are taken (I know because it turns off a bunch of lights and when I get back, those lights, which were on when I left, are off). But since they're off, I know that the "arrive home" scene hasn't triggered.

The trouble is that when I return home and the actions in the "arrive home" scene have not triggered automatically, I have to ask Siri to execute that scene.

I don't know if this is related, but I also have an iPad and an Apple Watch. Do these devices need to "leave" or "arrive" in order for the scene to trigger?

Anyone else seen this problem? And just what, exactly, is the trigger for leaving and arriving? When the iPhone loses/achieves connection with the WiFi the Apple Home hub is on? Do other things need to be true, like the Home app has to be open on the iPhone when it "arrives"?