Polework is generally a strengthen & conditioning exercise. It can help with working over the back, moving correctly and straightly, but it won’t be a magic tool to soften a horse’s mouth. Poles can also help with elasticity, which I’ll touch on later.
Have you tried working on leg yielding with either? I’ve actually found that to be very beneficial in the introductory stages, getting them to step over and really work over their back and subsequently reach for the contact. After about 10 minutes of leg yielding to the wall and figure eighting at the walk, I generally have a horse that is reaching down into the contact and working over his back. I do it at the walk mostly, getting them to step over without increasing their speed, and once I feel them really working from behind I’ll ask for a nice stretchy trot circle which is their mental “break” from the leg-yield work, keeping my outside rein on and gently opening my inside rein, while asking them to step to the outside rein each time I feel their inside hind about to come forward. It is way easier said than done, but timing your leg aid to their inside hind will help keep them from collapsing their inside shoulder and keep them moving forward and out.
People underestimate the usefulness of figure eights and serpentines, they really get the horse supple and elastic when ridden correctly, and then you can introduce working long and low from there. You have to start really small. In the beginning they only offer a few steps of long and low and you have to praise them heavily, take a break and work on something easier, and go back. I like to start by doing figure eights, and each time I go across the diagonal to change bend I open my inside rein for a little more flexion than usual combined with a push of my inside calf and seat to get them onto my outside rein - I’ll slowly get “greedier” with each passing lap, and they tend to understand what I am asking quickly, but it’s hard for them to do more than a few steps. I’ve found that asking them to change their bend while on a straight line is a challenging exercise that helps free them up and supple them – they’ll start reaching down into the bit and when they do that, be quiet for a moment in your aids. It’s harder than it sounds, as a lot of horses want to wiggle while you do this - you need to work on keeping them on a straight line but changing their bend fluidly.
For pole work and elasticity, I like putting 2 poles down at opposite ends of each other on a 20m circle (IE @ 12 and 6). I’ll ask them to trot, and work on getting them to go from medium to working between each trot pole. Same with canter. Initially they might rush, think about posting from the 3 seat bones and sitting each stride “longer”… don’t bring your hand back to slow them, instead, put your hand forward and leg on while controlling the tempo from your seat/posting.
For getting a bigger/better step in the stride, 4 trot poles in a row, I prefer on the centerline. Get them in a good trot rhythm and while they go over the poles, focus on how big they feel and try to keep posting in that “big” trot all the way to the end of the ring. See how long you can keep that feeling and keep the leg on over the poles. The goal is eventually for that suspension and fluid motion to be “held” all the way around the ring.
For conditioning, I love warming up over walk poles. I prefer 6 in a row, but if ring space is an issue, 4 is ok. I put them on the outside rail so that during walk breaks, we walk over them. I walk over poles daily.
These are harder exercises for a green or unconditioned horse, so I usually only ask for 20m of real work from them when doing this.