onAdjustVolume not called

Question

I am trying to get HARDWARE_HOOK button and volume change button in my service using MediaSession for Android 5.0+ . This has to work specially when screen is off/locked. The problem is that though I am able to receive HARDWARE_HOOK button event, the volume change is not detected. the onAdjustVolume method never gets called.

import android.app.Service;
import android.content.Intent;
import android.media.MediaRouter;
import android.media.session.MediaSession;
import android.os.Binder;
import android.os.IBinder;
import android.support.v4.media.VolumeProviderCompat;
import android.support.v4.media.session.MediaSessionCompat;
import android.util.Log;
import android.view.KeyEvent;
import android.widget.Toast;

public class HookButtonService extends Service{

    public static final String SESSION_TAG = "SampleApp";

    private MediaSessionCompat mMediaSession;

    private VolumeProviderCompat myVolumeProvider;

    public class ServiceBinder extends Binder {

        public HookButtonService getService() {
            return HookButtonService.this;
        }
    }

    private Binder mBinder = new ServiceBinder();

    private MediaSessionCompat.Callback mMediaSessionCallback = new MediaSessionCompat.Callback() {

        @Override
        public boolean onMediaButtonEvent(Intent mediaButtonEvent) {
            Log.d("SampleApp","Media button received");
            if(!MyApplication.isActivityVisible()) {
                Intent dialogIntent = new Intent(getApplicationContext(), MainActivity.class);
                dialogIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                startActivity(dialogIntent);
            }
            return true;
        }
    };



    public HookButtonService() {
    }

    public MediaSessionCompat.Token getMediaSessionToken() {
        return mMediaSession.getSessionToken();
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    @Override
    public void onCreate() {
        super.onCreate();

        mMediaSession = new MediaSessionCompat(this, SESSION_TAG);
        mMediaSession.setCallback(mMediaSessionCallback);

        myVolumeProvider = new VolumeProviderCompat(VolumeProviderCompat.VOLUME_CONTROL_RELATIVE, 100, 50) {
            @Override
            public void onAdjustVolume(int direction) {
                Log.d("SampleApp","Volume change received: "+direction);

            }
        };

        mMediaSession.setActive(true);
        mMediaSession.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS |
                MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);
        mMediaSession.setPlaybackToRemote(myVolumeProvider);

    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mMediaSession.release();
    }


}