Can only intermittently view my son's location in Find My iphone

My son's phone is set up on find my iphone. I used to be able to view its live location all the time. Recently I can't view it some of the time, and can't only ever view a static location - what usually happens is that I can view where it is when I log into findmy for the first time, but it doesn't track him moving. If I want to see where he's moved to I have to go out of the screen where I'm viewing his location back to the list of all the devices I'm tracking, then sometimes it updates and I can go back in and see another static location. Other times it won't update at all. I can see all my other devices in live time and they are tracking movement.

I've tried all the trouble shooting I can find online but everything I can find assumes findmy isn't working at all. I can't find a solution for it only intermittently working. Obviously the settings must be correct for it to be working at least some of the time. I've tried turning both phones on and off and closing apps etc and that doesn't make any difference.

I have screentime control settings set up on his phone, but I've checked everything and there doesn't seem to be anything getting in the way that is immediately obvious.

Any ideas?