Sometimes the reasons for a fall-off as someone's just beginning to take off are quite sad. One that springs to mind is Amy Studt, who - back in 2002 or 2003 - was beginning to take off as the UK's answer to the likes of Avril Lavigne:
All great songs. Then the label rereleased her album after the success of these singles - with just one new song (a Sheryl Crow cover) that didn't chart well, so she got dropped, and suffered a bunch of health issues - some mental - and also struggled with addiction.
Pretty sad how the music industry can just chew up and spit people out on a whim, especially when they're so young and vulnerable.