"Find My" stopped updating

My wife went to dinner with a friend of ours. Out of curiosity, I tried "Find My" on my phone. Never tried it before. It showed her at the restaurant where I expected her to be, but then stopped updating. Half an hour later she was back home, and I said, gee, did you turn your phone off or something? Your location wasn't updating. She said, um, I didn't do anything to it. She pulled her phone out, and it was on, and my "Find MY" STILL showed her at the restaurant and said "30 minutes ago". What's going on here? Why does an operational phone just stop updating location? This is NOT handy.