After 2 weeks in Master Luke Chan's retreat in China, I got a big improvement involving my lower back and Achilles tendon. 

I was never able to sit on my heels, with my feet together. I always lost my balance when I did that, because my lower back  and Achilles tendon was not loose enough. Holding a pole everyday for 15 minutes on my heels, with my feet together, my arm straight forwards at shoulder level and pulling myself towards the pole, until I couldn't go further forwards and then just holding this position gave a good result.

Now I am able to keep this position, without losing my balance.

My lower back is looser and it is easier to do the wallsquats closer to the wall.