android - NotificationCompat not grouping notifications -

i'm trying group notifications apparently keep coming separately.

notificationmanager mnotificationmanager = (notificationmanager) this.getsystemservice(context.notification_service);  intent resultintent = new intent(this, splashscreenbaseactivity.class); resultintent.putextra(arguments.arg_notification_type, news_type); taskstackbuilder stackbuilder = taskstackbuilder.create(this); stackbuilder.addnextintent(resultintent); pendingintent resultpendingintent =         stackbuilder.getpendingintent(                 0,                 pendingintent.flag_update_current         );  notificationcompat.builder mbuilder =         new notificationcompat.builder(this)                 .setsmallicon(r.drawable.ic_launcher_small)                     .setlargeicon(bitmapfactory.decoderesource(getresources(), r.mipmap.ic_launcher))                     .setcontenttitle(contenttitle)                     .setcontenttext(contenttext)                     .setautocancel(true)                     .setgroup(news_group)                     .setgroupsummary(true)                     .setdefaults(-1);  mbuilder.setcontentintent(resultpendingintent);  random random = new random(); notification_id = random.nextint(9999 - 1000) + 1000; mnotificationmanager.notify(notification_id, mbuilder.build());