A lot of issues with turing come from lack of forward. The horse must be thinking forward before they turn. If they decide to go a diffrent direction you will notice that the slow down or back off the turn. Ensure horse is forward and ask to turn. Do not keep pulling the head aroung this will not turn the horse but push the horse forward and open the rein in the direction you would like to go.
If none of these tips that these riders have given you then maybe try to clicker train him so like going back to the basics and reward him when he does the right thing you can read up on clicker training on the internet. I have attended a course before and whenever i struggle i go back to basics because if you can do it with your horse on foot you can do it while riding him/her. I am not a professional rider or anything so i would try the other tips first before mine
Your outside rein is your "wall" that keeps the horse from drifting. Maintaining steady, gentle contact on the outside rein is key to turning. You will want to use your inside leg to "push" the horse over. Gently squeeze and release repeatedly with the inside rein as a kind of "please and thank you." Your shoulders and torso should be facing the way you want to go-- If your chest is pointed the wrong way the horse won't go where you want them to! If you want to go right, your upper body should be turned to the right. Also try not to drop your inside shoulder, keep it up and back.
My coach always tells me to do the opposite of what the horse wants to do. So if your horse wants to go left, go right. If he wants to go fast make him collect himself. To make him turn the way you want it to, sit deep in the saddle, look at where you want to go, and use your reigns - bringing your hand to your hip. Hope this helps a bit!