Here are the things to check if you are facing this issue:
Make sure you are using a Listview/Gridview/Row/Column to dynamically generate the children.
Make sure your query is for a list of documents and not a single record.
If you are using a filter then make sure the data present in Firestore has more than 1 record that can pass through the filter.
Make sure that your Firestore collection has enough records.
If you are querying a single field then make sure it’s a ListType field in FlutterFlow and in Firebase.