Unless you engage the back of your legs u simply cannot do the basic locust I found out today. The lift is against gravity, with your own body dragging it down (unlike the bherundasana version where you can jump off that resistance to stay aloft). So, back of the legs need to be really worked. One way to start on it is through leg raises that work the butt. Also, engage the foot while doing even the basic leg lift. Most people focus on the leg and let the foot hang loose. Focusing there will help engage the back of your legs also.