After a T-Mobile update to Android Pie, my Always on Display was no longer displaying. It seems like in Pie it initially defaults to only displaying with a touch to the screen. To change this, go to Settings, Lock Screen -> Always on Display – some of these setting screens have additional options if you scroll down. For example, the AoD settings page initially looks like this:
… but if you scroll down you’ll see additional options, including ‘Display Mode’:
Press Display Mode and change from ‘Tap to Show’ to ‘Show Always’ :
Done!