Retrofit object returned as null

I've built a RetroFitService to return some objects. In MainActivity I call the service with a simple button click. I seem to be getting some kind of object, but I don't feel it's actually being returned from the REST API that I specified. It shows up in the debugger but its attributes are null:

bFetch.setOnClickListener(v -> {
 v.startAnimation(AnimationUtils.loadAnimation(this, R.anim.image_click));
 RetrofitService service = ServiceFactory.createRetrofitService(RetrofitService.class, RetrofitService.SERVICE_ENDPOINT);
 service.getPosts()
 .subscribeOn(Schedulers.newThread())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(new Subscriber < Post > () {
  @Override
  public final void onCompleted() {
   Log.e("RetrofitService", "Retrofit Request Completed!");
  }

  @Override
  public final void onError(Throwable e) {
   Log.e("RetrofitService", e.getMessage());
  }

  @Override
  public final void onNext(Post post) {
   if (post != null) {
    // TODO: Some object is returned but its properties are null
    Log.e("RetrofitService", "Returned objects: " + post);
    Log.e("RetrofitService", "Object Id: " + post.getObjectId());
    mCardAdapter.addData(post);
   } else {
    Log.e("RetrofitService", "Object returned is null.");
   }

  }

 });

});

}

Service:

public interface RetrofitService {

 String SERVICE_ENDPOINT = "https://parseapi.back4app.com/";

 @Headers({
  "X-Parse-Application-Id: asdf",
  "X-Parse-REST-API-Key: asdf"
 })
 @GET("/classes/Post")
 Observable < Post > getPosts();

 /*curl -X GET \
         -H "X-Parse-Application-Id: asdf" \
         -H "X-Parse-REST-API-Key: asdf" \
 https://parseapi.back4app.com/classes/Post*/

}

The curl works just fine. I'm not getting any errors. What could be going wrong? Is my @GET method incorrect somehow?`

For completion, here is the ServiceFactory class:

public class ServiceFactory {

    /**
     * Creates a retrofit service from an arbitrary class (clazz)
     * @param clazz Java interface of the retrofit service
     * @param endPoint REST endpoint url
     * @return retrofit service with defined endpoint
     */
    public static <T> T createRetrofitService(final Class<T> clazz, final String endPoint) {
        final RestAdapter restAdapter = new RestAdapter.Builder()
                .setEndpoint(endPoint)
                .build();
        T service = restAdapter.create(clazz);

        return service;
    }
}

And my build.gradle because I'm aware that there are inconsistencies across all the different Retrofit versions:

dependencies {

    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.3.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'

    /* ReactiveX */
    compile 'io.reactivex:rxjava:1.0.17'
    compile 'io.reactivex:rxandroid:0.23.0'

    /* Retrofit */
    compile 'com.squareup.retrofit:retrofit:1.9.0'

    /* OkHttp3 */
    compile 'com.squareup.okhttp3:okhttp:3.8.1'

    /* RecylerView */
    compile 'com.android.support:recyclerview-v7:25.3.1'

    /* CardView */
    compile 'com.android.support:cardview-v7:25.3.1'

    /* Parse */
    compile 'com.parse:parse-android:1.13.0'

}

Post Class:

public class Post implements Serializable {

    private static final String CLASS_NAME = "Post";

    private String objectId;
    private String text;

    public Post(String objectId) {
        this.setObjectId(objectId);
    }

    public static String getClassName() {
        return CLASS_NAME;
    }

    public String getObjectId() {
        return objectId;
    }

    private void setObjectId(String objectId) {
        this.objectId = objectId;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

Curl Response:

>     https://parseapi.back4app.com/classes/Post/
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   259  100   259    0     0    360      0 --:--:-- --:--:-- --:--:--   395{"results":[{"objectId":"ktEfgr1pFt","text":"Hello World.","createdAt":"2017-08-14T14:07:52.826Z","updatedAt":"2017-08-14T14:07:52.826Z"},{"objectId":"Mmh8l9gjCk","text":"Hello?","createdAt":"2017-08-14T15:19:01.515Z","updatedAt":"2017-08-14T15:19:03.743Z"}]}

FINAL UPDATE: I changed the onNext() method of the RetrofitService to pass into the CardAdapter, although that isn't shown here and surpasses the scope of the question.

@Override
public final void onNext(PostResponse postResponse) {
 if (postResponse != null) {
  // TODO: Some object is returned but its properties are null
  Log.e("RetrofitService", "Objects successfully added to RecyclerView Adapter.");
  Log.e("RetrofitService", "Returned objects: " + postResponse.getResults());
  Log.e("RetrofitService", "Text " + postResponse.getResults().get(0).getText());

  mCardAdapter.addData(postResponse);
  //
 } else {
  Log.e("RetrofitService", "Object returned is null.");
 }

}