The reason you are not able to do padmaasana are many and the answers may not fully satisfy you till I see you but I can try to explain why: Sitting crossed legged, on the floor especially, can be difficult for some. It is a function of a number of muscle groups namely the hip flexors(in the front of the hips, as they join the legs), the muscles in the buttock, and the hamstring group behind the thighs, as also the muscles in the lower back. So strengthening AND adequately stretching each group (along with the rest of body!) may help alleviate the problem.