I haven't played in a while and the game may have updated since but are you picking girls for each song when you do concerts? The most popular girls always give the biggest boost.
When you add a song to the concert, the game seems to automatically pick whatever girl was center when you made the single. I don't know if you can change it, so I mostly had my "best" idol lead almost every single until I had a couple of successful songs. My first concert only pulled in maybe ¥300 000 profit, but a few months later I had some songs sell well enough that my second concert pulled in maybe ¥19 000 000 in profit. I'm still not quite sure how it all works.