If others can do it, and you, in same tack and all, cannot, then it must be in your riding. could be your hand is either too hard, or not firm enough, or timing, or that you are gripping up with your heels, or bumping down on his back, or leaning forward, or any number of things that are just different from the other rider to be ineffective.
I can get short moments of decent engagement and lateral movement, for one or two steps, while my trainer, in same tack on same horse can have him do this consistently. It's most certainly that I am not balanced enough, am too busy with my hands, miss the correct timing for a release, or, "throw" the horse away, and on and on. The only time I improve is through lessons. otherwise, I simply keep repeating what feels right to me, but is NOT.