For me, the biggest hurdle to the backwards roll was the lack of momentum miss Giri mentioned .
It is actually MUCH harder to roll backward from a *static* position (kneeling) than from a standing start. The step back to lower your hip towards the mat, is where you gain a lot of the needed inertia.
It is great to use the "thrown arms" she described as well.
Are you having trouble when you are standing and starting? Or is it just when starting from a kneel?
Rolling will come. *next is breakfalls*