Elbows behind the knee?

Hi,
Here I am, 20+ classes into Bikram and no idea how to get my elbows BEHIND my knees in the padahastana/ forward bend…i am unable to cup my hands from behind and am getting little direction in class…looking at the expert front row: everyone has it down…how can i get there? help please!

Note that i have a ‘larger’ bust area, so bending my legs more only helps so much.

thanks!