The world of mobile gaming is finicky. You can have a ton of downloads but they’re free, so if you don’t have the good quality gameplay to go with it, money will not reflect this.
According to SEGA Networks this has been the case for Sonic Runners. They consider it a “stalling title” but more specifically, a “failure”.
The game makes ¥30 million [$256,563 USD] to ¥50 million [$427,588 USD] a month. ¥50 million [$427,588 USD] is also the amount that SEGA considers a title to be doing well, so it’s really close. But it’s just not enough.