Think of this experience as an opportunity to establish a language with the horse. While you may want a horse to understand certain cues, you may likely need to begin with something a little easier for the horse to understand and transition from there.
For example, I usually get a horse to turn by simply drawing my outside leg back an inch or so and rotating my upper body in the direction I want to go. This brings a lot of things into play including the outside rein touching the horse's neck. On the few occasions when that doesn't work. I might take my inside hand out to the side to help draw the horse's head in that direction. I would release any pressure the moment the horse gave an indication of a response. In that way, I could repeat it if necessary. I would not, however, be pulling the horse through the turn.
One particularly sensitive horse didn't even seem to understand that. When I touched his withers with my outside hand, however, he turned. I used this action to help him understand what my other actions had been meant to communicate.
Another example is the leg-yield. I begin this work at a walk so the horse has more time to think and respond correctly. Rather than steady pressure with the inside leg. I "pulse" this pressure with the timing of the horse's inside leg stepping forward. As the horse lifts this foot off the ground to move it forward, my leg is telling him: "Let's move it sideways just a little bit as well." The other legs will naturally follow the diagonal motion of the first.
Think about how you asking and how the horse might be understanding. If the horse doesn't seem to understand, don't simply shout the same thing more loudly. Try to find a different way of explaining what you want.
Another important thing to remember is not to inhibit the horse from doing what you want. Riders often concentrate on one thing they are doing and don't realize how other things they are doing at the same time may be preventing the horse from doing what they want. Worse yet, is when the horse thinks the rider's other action is telling him to do something else.
An example of the latter is the rider who draws his leg several inches away from the horse's side in order to "open the door" and get the horse to move in that direction. To keep balance on the horse, the rider shifts his weight to the opposite hip. Rather than moving in the direction of the "open door", the horse tries to keep his rider from falling off by moving in the direction of the excess weight.
I wrote an article once about a teacher who worked with children with learning disabilities. He explained that the difficulty was often one of perception. The children weren't stupid. The teacher just needed to find the correct method of communicating.
Since every person and every horse are individuals, there is no one magic cue. A broad knowledge of methods that have worked for others with other horse provides the rider ready variety of things to try. But the rider should also be open to experimentation. Try something and be sensitive to how the horse responds. Alter your next attempt accordingly.