MongoDB and Java: Query doesn't return results

I'm using the MongoDB Java driver, and can't seem to get a query to work. I have a collection named "Questions", which has entries that look like:

{
    "question"  : "how are you",
    "category"  : "personal",
    "processed" : false,
    "training"  : true
}

When running the Mongo command line client, the query

db.questions.find()

or

db.questions.find({"processed" : false, "training" : true})

results are returned as expected; however, my java code does the following:

DBObject queryObj = new BasicDBObject();
queryObj.put("processed", false);
queryObj.put("training",   isTrain);

DBObject updateObj = new BasicDBObject();
queryObj.put("processed", true);

DBCursor cursor = mongoCollection.find(queryObj).limit(NUM_TO_LOAD);
mongoCollection.update(queryObj, updateObj);

and the cursor that is returned to me is empty/the update doesn't make any changes. If I remove the queryObj argument from the call to find, results are once again returned as expected. Am I doing something wrong here?

Thanks,
Chris Covert