Mongo db will not update an existing Document

Question

I am using java pojo's and spring MongoTemplate, I spent few hours now, trying to understand why MongoDb will not update an existing Document
I am able to insert documents, but not to update existing ones
Any simple way to trace this in mongo/spring?
I tried so far:
Simply call save:

        person.setUpdateDate(new Date());
        mongoTemplate.save(person);
        //not working

call updateFirst

        mongoTemplate.updateFirst(new Query(Criteria.where("_id").is(person.getId())),
                                    Update.update("objName", "NotWorking"),Person.class);

Get the collection first, convert to DB Object, then find and modify:

        DBCollection coll=mongoTemplate.getCollection("Person");
        DBObject mongoPerson = morphia.toDBObject(person);
        DBObject afterSave = coll.findAndModify(new BasicDBObject("id", person.getId()), mongoPerson);
        //afterSave is null

Try find by _id, then save:

        DBObject mongoPerson = morphia.toDBObject(person);
        Person found1 = mongoTemplate.findById(person.getId(), Person.class);
        found1.setObjName(person.getName());
        mongoTemplate.save(found1);

Find first, then save using "_id":

        DBCursor found = coll.find(new BasicDBObject("_id", person.getId()));
        DBObject first = found.next();
        mongoTemplate.save(first);

Find first, then save using "id":

        DBCursor found = coll.find(new BasicDBObject("id", person.getId()));
        DBObject first = found.next();
        mongoTemplate.save(first);

Try find and modify then save:

        DBObject afterSave = coll.findAndModify(new BasicDBObject("id", person.getId()), mongoPerson);
        //afterSave is null
        mongoTemplate.save(person);

Update individual fields using query:

        Query query = new Query();
        query.addCriteria(Criteria.where("_id").is(person.getId()));
        mongoTemplate.updateFirst(query, new Update().set("name",person.getName())
                                                     .set("desc", person.getDesc())
                                                      .set("updateDate",new Date())
                                                          , 
                                                        Person.class)

Answer 1

*Update this is also working:

mongoOperations.save(person,"Person");

Using this article I was able to make update work - not the way I want, but...It works...

import static org.springframework.data.mongodb.core.query.Criteria.where;  
import static org.springframework.data.mongodb.core.query.Query.query;  
import static org.springframework.data.mongodb.core.query.Update.update; 

@Controller
public class PersonController {

@Autowired
private MongoTemplate mongoTemplate;

@Autowired
MongoOperations  mongoOperations;

...

mongoOperations.updateFirst(query(where("_id").is(person.getId())), Update.update("name", newPersonName),"Person");  

Not sure yet why this work while

mongoOperations.save(person);

Is not working...