"No location found" for friend in Find My on iPhone

A few weeks ago, I was able to see my friend's FIND MY location but just last week, even when his location settings and mobile data settings are all on, I still couldn't see his location unless he open the app then I would be able to see but once he closes his app, I wont be able to locate him anymore. The states on my side would show that there is "no location found" but on his side, he could see mine very clearly even if I have close my app. Why is this and how could we slove this? Because it doesn't make sense that it was working very well a few weeks ago but then last week it just stopped when none of us pressed anything towards the FIND MY app.

[Re-Titled by Moderator]