MongoDB Permissions – Denying API Access While Console Shell Works

I am a MongoDB newbie, with a MongoDB instance running on a Ubuntu machine.

If I login onto the machine's console and run mongo -u username -p password databasename I have permissions for the database databasename to create and drop collections, insert documents and remove them. I have successfully tested these operations.

However, if I try to insert a document (into the same database using the same user credentials) using the MongoDB API from a Java program (running on the same server), I get error 13:

com.mongodb.MongoCommandException: Command failed with error 13: 
'not authorized on databasename to execute command { insert: "collectionName" ...

My Java code (works well when running on my local machine for another MongoDB instance without authentication):

...
ServerAddress serverAddress = new ServerAddress(serverName, portNo);
MongoCredential mongoCredential = 
     MongoCredential.createCredential(username, DB_NAME, password.toCharArray());
mongoClient = new MongoClient(...
DB db = mongoClient.getDB("databasename");
mongoCollection = db.getCollection("collectionName");
this.mongoCollection.insert(...                          <-- CAUSES EXCEPTION
....

What could be the cause of this error, and how can I solve it?