Я использую этот код в BroadcastReceiver:
public class NotificationReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent) {
NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
Intent repeating_intent = new Intent(context, MainActivity.class);
repeating_intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
PendingIntent pendingIntent = PendingIntent.getActivity(context, 100, repeating_intent, PendingIntent.FLAG_UPDATE_CURRENT);
StringBuilder sb = new StringBuilder();
NotificationCompat.BigTextStyle contentStyle = new NotificationCompat.BigTextStyle();
contentStyle.bigText((CharSequence) sb.toString());
NotificationCompat.Builder builder = new NotificationCompat.Builder(context) // channel ID missing
.setContentIntent(pendingIntent)
.setSmallIcon(R.drawable.icon)
.setStyle(contentStyle)
.setContentTitle("Title")
.setContentText(sb.toString())
.setAutoCancel(true);
notificationManager.notify(100, builder.build());
}
Я думаю, вы забыли последнюю строку:
notificationManager.notify(100, builder.build());