I found that achieving a free walk with my arab, the key was to relax MYSELF. My whole position changes when I ask for a free walk/long and low. A couple strides before I ask, I sit taller in the saddle and really engage my core, then when I ask I breathe out and just relax and let myself drop a little and Bob's your uncle, I have a beautiful free walk.
I used to sit there stressing about getting him to relax and why is this technique and that advice not working, then me friend told me to just calm down about it, focus on keeping the walk active with pendulum legs and give slightly with my hands each step and RELAX and it will come. And it did :)