If you have a non expert horse he should learn that if he moves away from a physical pressure rather than resisting it, the pressure goes away. In reality he can relieve a pressure by moving forward away from it or he can relieve it by moving sideways away from it. Moving sideways away from a pressure results in the reward of the pressure going away, this is absolutely fundamental.
Also it is important giving the horse the time to learn and adquire all this knowledge. Starting doing it at walk may be a good approach. Also a single movement with a good response is preferable than a long imprecise diagonal.
Good evening Zoe!
There are some exercises that help me a lot when trying to unlock my horses. Starting with hips, a leg-yield is always a good option because it moves 50/50 forward and sideways the horse loosening your horse's shoulders, back and hips.
Shoulders are also important, starting at the walk and then trotting, make a square instead of a circle, riding your horse with a counterflexion to the outside. At each corner, move your hands to the inside to swivel your horse's shoulders around the 90 degree turn.
For more suppleness in the neck i use to ride on a circle, using the inside rein in the same manner as unlocking the poll, but asking for a bigger bend. It is important to do it quickly but smoothly.
Hope this helps a bit :)