Have been signed up for about 5 years now.
If you order off your phone app, make sure it's still active. They boot you out on mobile once a year and make you reactivate. It's not a fully set it and forget it thing. That's not a problem on the desktop website though.
In general if you turn this on via the website then you have to also activate on you phone. It won't automatically do it. I assume Amazon knows more purchases take place on mobile and it's their way of trying to save a buck.