My TB refused to do the stretch..it was just his nature...he preferred head up or level in the walk even though he would take the rein and stretch OUT..he just wouldn't go down. Essentially I trained it into him by asking with the reins to lower his head while still asking for a more active walk. Once I had a better head position I would slow the walk to get the "march" the judges like to see and get swinging through his back again. As time went on and he got more used to the dressage frame, around Training Level, he was more willing to go down and out after the first few test elements. The first show ever at Intro A and B he was getting 5s...by Training 2 he was getting 8s in his walk but we still had the head level issue in the stretchy trot.
The other option is to get a pal with a small pony, tie a carrot to the pony's tail and have your horse follow :)