Phone not Auto-Locking

Hi, I have had my iPhone xR since it was released back in October. I am having a problem where my phone will not Auto-Lock, even though I have it set to lock after 30 seconds. About once a week, I will turn my phone off and then back on and it will start Auto-Locking again. However, it is getting annoying to have to do this. Is there anything I can do to help? It is updated with the most recent software.