Starting on hills hits noobs with the ol' double whammy. Balancing at low speed is tough anyway, but then the new rider is still learning how to start without pushing off like they'd do on an upright. Make sure you're in a low gear, make sure you start with your strongest leg in the 12:00 or even a little earlier position. Push hard and relax the upper body. With enough practice and a low enough gear, you can even start on 15+% grades.