Actor position is incorrect - Libgdx

Question

Please help me, I'm pulling my hair out.

I have a class called "HudBarView" which extends the Group class(scene2d). It takes care of the drawing of the HUD Bar at the top of the screen(Pause button, score, etc).

Another class I have is "HUDManager" which takes care of all the HUD work in the game, including HUD Bar.

These two classes are very short and self-explantory. There is nothing sophisticated.

Now, currently I have only a pause button(ImageButton) in my HUD Bar but the problem is - it is visible only after I resize or pause and then resume the screen.

Here is a gif that describes my problem: http://i.gyazo.com/6140c4ac1ea0778e4e1afce161ea3dc0.gif

I have tried to play a little bit with the code and I found out that the positioning of the actor is wrong. For instance if I define the position of the button with these values:

Vector2 position = new Vector2(Values.SCREEN_WIDTH-Values.Pause_Width*2,Values.SCREEN_HEIGHT-Values.Pause_Height*2);

Instead of these:

Vector2 position = new Vector2(Values.SCREEN_WIDTH-Values.Pause_Width,Values.SCREEN_HEIGHT-Values.Pause_Height);

It does show up on the screen but at the wrong position and after pause/resume it show up at the correct position.

Here is my HudBarView class:

    package views.hud.views;

import views.renderers.HUDManager;

import com.badlogic.gdx.graphics.g2d.TextureAtlas;
import com.badlogic.gdx.scenes.scene2d.Group;
import com.badlogic.gdx.scenes.scene2d.ui.ImageButton;
import com.badlogic.gdx.scenes.scene2d.ui.Skin;

import engine.helpers.AssetsManager;
import engine.helpers.Values;

public class HUDBarView extends Group{

    private ImageButton pause;
    private HUDManager hud_manager;
    private Skin skin;
    public HUDBarView(HUDManager hud_manager) {
        this.hud_manager = hud_manager;
        initiate();
    }
    private void initiate() {
        skin = new Skin(AssetsManager.getAsset(Values.BUTTONS_PACK, TextureAtlas.class));
        pause = new ImageButton(skin.getDrawable("pause"));
        pause.setSize(Values.Pause_Width, Values.Pause_Height);
        pause.setPosition(Values.SCREEN_WIDTH - pause.getWidth(), Values.SCREEN_HEIGHT- pause.getHeight());
        addActor(pause);
    }
}

HUDManager class:

package views.renderers;

import views.hud.ParticleEffectsActor;
import views.hud.views.HUDBarView;
import aurelienribon.tweenengine.TweenManager;

import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.g2d.TextureAtlas;
import com.badlogic.gdx.math.MathUtils;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.scenes.scene2d.Group;
import com.badlogic.gdx.scenes.scene2d.Stage;
import com.badlogic.gdx.scenes.scene2d.ui.Image;
import com.badlogic.gdx.scenes.scene2d.ui.Label;
import com.badlogic.gdx.scenes.scene2d.ui.Skin;

import engine.helpers.AssetsManager;
import engine.helpers.UIHelper;
import engine.helpers.Values;

public class HUDManager extends Group {

    private Stage stage;
    private Skin skin;
    private Image text;
    private HUDBarView hudView;

    public HUDManager(Stage stage) {
        this.stage = stage;
        initiate();
        addActor(hudView);
        addActor(particles);
    }

    public void initiate() {
        skin = AssetsManager.getAsset(Values.GAME_SKIN_PACK, Skin.class);
        hudView = new HUDBarView(this);
        particles = new ParticleEffectsActor();
    }

    public Stage getStage() {
        return stage;
    }
}

And Here is my game screen problem:

package views.screens;

import views.renderers.GameRenderer;
import views.renderers.HUDManager;
import engine.GameInputHandler;
import engine.GameWorld;
import engine.Values;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.InputMultiplexer;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.scenes.scene2d.Stage;
import com.badlogic.gdx.utils.viewport.StretchViewport;

/* Created by David Lasry : 10/25/14 */
public class GameScreen extends ScreenWrapper {

    private GameWorld world;
    private HUDManager hudManager;
    private GameRenderer renderer;
    private Stage stage;

    public GameScreen() {
        super();
        ScreenHandler.getInstance().getGame().getAssetsManager()
                .loadAsstes(2, this);
        world = new GameWorld();
        stage = new Stage(new StretchViewport(Values.SCREEN_WIDTH,
                Values.SCREEN_HEIGHT));
        camera = (OrthographicCamera) stage.getCamera();
        InputMultiplexer inputs = new InputMultiplexer();
        inputs.addProcessor(stage);
        inputs.addProcessor(new GameInputHandler(world.getMonkeyManager(),
                camera));
        Gdx.input.setInputProcessor(inputs);
    }

    @Override
    public void render(float delta) {
        switch (state) {
        case RUN:
            stage.act(delta);
            stage.draw();
            world.update(delta);
            break;
        case PAUSE:
            break;
        }
    }

    // Callback method that is called when the assets are loaded
    @Override
    public void assetsLoaded() {
        super.assetsLoaded();
        renderer = new GameRenderer(world);
        hudManager = new HUDManager();
        stage.addActor(renderer);
        stage.addActor(hudManager);
    }

    public HUDManager getHudManager() {
        return hudManager;
    }

    @Override
    public void dispose() {
        stage.dispose();
        ScreenHandler.getInstance().getGame().getAssetsManager().unload(2);
    }
}

I'm struggling with this problem for almost 3 days and I have no idea what is causing this. I even opened a new project, duplicated the relevant classes and just tried to position the button at the top right corner of the screen and it worked! So why it doesn't work in my actual game?

Please help !