Simple Automation Not Working - ST & Hue

I have a very simple automation set up in SmartThings to turn two Hue bulbs on when I arrive home, based on proximity of my phone. For the last two days, it has failed to turn them on. It happened once before some weeks back.

I’m assuming for this to work, the phone’s location services must be on and so I’ve kept it on 24/7 since setting up the system, although it drains the phone battery twice as much. Only when I open the SmartThings app to check on the automation settings to make sure they are still set (which they are) does the action take place and the lights turn on. It is as soon as I click the setting “When someone arrives” that the lights turn on.