1. 检查通知设置：在Android Studio的“File”菜单下选择“Settings”，然后在左侧边栏中的“Appearance & Behavior”选项中找到“Notifications”，检查您想要接收通知的选项是否已启用。

2. 检查通知频道设置：从Android 8.0开始，必须设置通知渠道以便正确显示通知。在您的应用程序中，通过以下代码设置通知渠道：

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
   NotificationChannel channel = new NotificationChannel("my_channel_id", "MyChannel", NotificationManager.IMPORTANCE_DEFAULT);
   NotificationManager notificationManager = getSystemService(NotificationManager.class);
   notificationManager.createNotificationChannel(channel);
}

3. 检查候选发件人列表：有时，您的通知可能被归类为垃圾邮件或任何其他不需要的文件夹中。检查您的候选发件人列表（例如Google等），以确保您的通知不被过滤。

4. 检查后台进程限制：在Android 8.0及更高版本中，系统会限制应用程序在后台运行的时间和资源。如果您的应用程序在后台运行，并且通知未及时发送，则可以通过以下代码解除限制：

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
    // Create the NotificationChannel, but only on API 26+ because
    // the NotificationChannel class is new and not in the support library
    NotificationChannel channel = new NotificationChannel(CHANNEL_ID,
            "Channel human readable title",
            NotificationManager.IMPORTANCE_DEFAULT);
    channel.setDescription("description text");

    // Register the channel with the system; you can't change the importance
    // or other notification behaviors after this
    NotificationManager notificationManager = getSystemService(NotificationManager.class);
    notificationManager.createNotificationChannel(channel);

    //start the foreground service to keep it running in background and not get killed by the system