Find My on my son's phone - why does it drop out?

I'm not completely up to speed on how Find My works s I feel like this may be a daft question. My son's account is connected to mine via the family settings but it often disappears from Find My and I don't know why that would be. He swears blind he's not adjusting any settings but I'm sure there must be something that I've either not set up correctly or granted access to that's preventing it from showing up. What can cause these drop outs and what can I do about them?