I've been using a XT910 for about one year and working with Smart Action all this time.
One of my favourite Actions is night offline. So using the triggers display off, time range and motion detection is switching on flight mode.
This worked a long time but suddenly (a couple of days or maybe weeks) this stopped.
So I created an action just with trigger motion detection and a message action. But this action was almost never activated.
I don't know the range of sensor values but if I look at CPU-Z all of the three values of the accelerometer jumps by 0.2 m/sē although the phone is on the table.
In rare situatiation the trigger works. Maybe my phone suddenly "hypersensitive", but I don't know why. The last update to Android 4.1.2 (original Motorola en.EU) is a couple of month ago.
Maybe you have an idea!