Having the same problem, have to remove/drop the collection into which was trying to import the documents
only then mongoimport started to work and
the documents appeared in the database.

Actually, at first have run mongoimport with the particular set of JSON file documents, but into the wrong database. Then all subsequent mongoimport calls with those files (but this time into the right database and collection) were not effective and have not added to the specified db/collection for no apparent reason.

Try to drop collection, and also maybe save any existing data that otherwise would be lost,
and then re-run mongoimport again