Well, if you look at the implementation of PushPlugin, you'll see that they use a constant notification ID:
public static final int NOTIFICATION_ID = 237;
...
mNotificationManager.notify((String) appName, NOTIFICATION_ID, mBuilder.build());
This would cause every new notification to overwrite the existing one. That's why only the last notification is displayed.
If you want to change the behavior, you have to change the code to use a unique notification ID for each notification.