Hey all. So I just struggled through to a solution for this by combining a few things I found on this forum and elsewhere. My wife and I had recently upgraded to new phones and location sharing was working before but after getting the new phones she could see my phone on the app but I couldn’t see hers. No amount of deleting and re adding or resetting helped. Even creating a “family” that the two of us were sharing locations within, did nothing to help.
The solution I found was actually three parts. I had seen someone mention that having older phones on their account caused a problem. So first thing I did was tap the “devices” tab in the Find My app. On my phone (the one that could be seen but not see) all the devices shown were active devices (devices I currently own and use). However on my wife’s phone (the one that could see but not be seen) it showed two of her older phones that she had never removed. We removed those from the “devices” list in her Find My app.
However that only removes the devices from the app, the next step after some digging was to go into the Apple ID settings which is the very first option in settings. It should be the one with your name. Once in there scroll down to see the list of devices associated with your Apple ID account. Remove all devices from your account that are no longer active (old phones that are gone etc).
Now at this point it *still* didn’t work, but then I figured out the last piece to the puzzle. Below your Apple ID settings in the main settings menu, (or below “family” if you have that active) will sometimes appear another “setting” that will have one or more notification badges. In here you’ll find all the annoying things Apple tries to make you do like upgrade to iCloud+ etc. On my wife’s phone she had a few badges but specifically she had one that read “resume data sync”. She said that it kept giving her an error when she had tried to click it in the past. However this time, after we had removed her old inactive phones from her iCloud account, it actually just worked when we clicked it. When clicked it prompted her to enter a passcode for one of her old iPhones (one we had just removed). Once that passcode was entered, the option to “resume data sync” disappeared and magically the Find My app started working again.
if you’ve been fiddling with the settings in the Find My app like removing and re adding the person, you may have to double check all the basics are set up again in both phones before it works, but I think this solution might actually be helpful to some people.
TLDR, remove any old unused devices from your iCloud accounts and clear any of the settings notifications where the badges appear just below the iCloud settings in the main settings menu.
Best of luck!