This would be indicative of an issue with your partners iPhone, not yours. Turn each step off and back on, then test.
- Tell your partner to go to Settings > [your name] > Find My > Make sure Share My Location is on. Go to Find My iPhone > Make sure it’s on.
- Go to Settings > Privacy and Security > Location Services > Make sure that’s turned on. Scroll down to Find My > Make sure it’s set to While Using the App.
- Lastly remove each other from the Find My app then add each other back.
If it still doesn’t work likely that your partner is in an area where their location cannot be transmitted as easily. Find My does rely a combination of signals from GPs satellites, cell towers, Wi-Fi networks and other connected devices nearby.