Similar thing happened to me a few months ago. I kept checking with UA agents to make sure my baggeage was with me through my one connection, and was repeatedly told it was. Arrived at destination with a txt message saying that my baggage didn’t make the flight.
Always best to be prepared for stuff like this. Make sure your daughter has her luggage tags, so she can head to the UA desk if her stuff didn’t make it. And keep all receipts.