When most people first attempt the fixed firm posture, they are unable to sit on their heels. It can take weeks or longer just to get the hips on the floor, and it might feel like you just conquerred Mount Everest when you do. Sound familiar?
If you want to improve this pose faster, Bikram recommends practicing it at home. You can sit Japanese style while watching TV. This will help you to stretch the muscles and tendons in your knees and feet. Please keep your feet straight and your toes pointed straight back. The tops of your feet should be on the floor. Your feet should be opened only wide enough so that your hips can touch the floor.
Once you are able to start going back, you can open your knees as much as you need to. Go back very slowly in the beginning to avoid pulling anything. This is a backbending posture, so it can be quite intimidating to drop your head back to the floor for the first time. If you have fear in the posture, the solution is to relax and breathe slowly. Go in to the posture slowly and come out of the posture slowly. It’s not about speed or what the posture looks like, it’s about healing the body. Remember the childhood fable; the tortoise wins the race.