You probably need to re-evaluate how you ride. Flexion at the poll (your goal) comes from the legs not the hands. You probably need to use more leg. I agree with the person above about the dressage trainer. Even if you are a hunter, jumper or equitation rider, especially the last two, you will benefit from one. To get a horse to round and move correctly, you first need to establish forward impulsion, energy, rhythm and tempo. Then by keeping a constant pressure on the outside rein and asking for inside bend with the inside rein, you can use your leg to encourage your horse to move into your outside rein. You cannot force your horse to round or put its head down. You need to encourage it to relax. Rounding comes from the back, not the neck. Often when a horse is putting its head up its goal is to protect its back from a bouncy or unbalanced rider. (Ensure you do not have either of those problems first or your horse with not soften and round). If you are really desperate, you could try draw reins, but the issue with that is that they force the horse’s head down. Horses often get nervous if someone forcefully controls their heads and your horse may not relax, expose and round his back. It’s much better if you can naturally encourage your horse to relax and round in a positive way. Personally, I only use a chambon or draw reins if I have a problem controlling my horse when it puts its head above the angle of control. I would recommend using a running martingale that will prevent your horse from raising new his head above the angle of control but allow him to stretch forward.