In 1.8.X, by the default, the frist blog will aggregate the posts from all other blogs. But new version 2.4.x, it won't do by default. I read some posts in forums. Some people suggested to use multiblogs.php and summary.php. Yes, they are right, if you want to do a custom layout, please use multiblogs.php, if you want to have a summary of your blogs, using summary.php is a right choice. But if you just want the frontpage working like the older version. Please go to backoffice->Blog setting. And select you default blog. At Advance tab, there is an option to set aggregate blogs' post. You need to enter ID of blogs need to be aggregated in a comma-separated format, such as "2,3,5". Then it will work for you.