Weird... Iīve tried to WILD just normally on my back for... lets say 50 times. I remember that every time my arms went to partial SP, starting from my fingers, but the rest of my body didnīt SP at all, exept for my toes.
Now Iīve discovered a better posture: I lay on my back, and I put my hands above my head. This way I donīt feel the urge to move.
After changing the posture, Iīve taken a huge step forward. I can almost always get my hands to SP. But thatīs it. My right hand goes to 80-90% SP, and the left to 30-60%. My toes go just a bit numb, and my face too, but just a few percents.

So in a nutshell:
Only my arms go to SP, expecially when my hands are above my head.

Any idea how I could get a FULL SP?