You may be expecting too much too soon. I began the transition to neck reining when I moved my horse from a snaffle to a curb bit. He was already working on a loose rein with the snaffle, just a touch of pressure on the inside rein to turn him into that direction. I consider Sam very quick and smart, but it still took us several practice sessions before he put the cues together. You may need to break it down into smaller learning chunks....
When I transitioned him to a curb bit the training process I followed included several cues.
To turn right (picture a right angled arrow):
Add: remember "home" position. When going straight (neutral) the reins should be in the inside hand and centered over the withers. This is important because the horse has to know when you are changing course. In "neutral" position, they are low to the withers, the reins are noticeably slack, and your hands are centered over the withers. This is the most important cue. This way when you move to actually cue, it makes an impression (opposed to your hands being all over the place).
1. With the split reins in my right hand, I placed the rein way up on the neck. That was my trainer's lesson to me; in the training phase the reins have to be up higher on the neck, just below the poll. Picture laying the left split rein across the neck about 2 inches below the poll. The right split rein must be slack, but able to create tension when asked. That is cue #1.
2. Cue #2, When you raise the left rein to the higher point of the neck, using your right hand pinky, tweak the slack right rein. This is the direct rein signal, so you are providing the new signal and providing the usual cue. As soon as the horse turns into the right rein, drop this cue. Release the neck rein pressure when the horse commits to turning.
You have to hold the reins properly to be able to tweak: Make a vertical cup with your hand, thumb on top. Reins fit between thumb and forefinger and down across palm. Stick pinky finger over right split rein, so you tweak the rein by pushing down on it.
If your horse needs a bigger signal, you CAN hold the reins in the left hand, bring across your body (top of horse) and use the right hand to provide more direct rein pressure. Watch your body however. Seat, weight distribution, etc. That can provide a conflicting cue to your horse.
3. If you are coordinated, you can use the left leg to provide a third cue. Begin pressing into your horse when your arms first move and release pressure when you stop tweaking the right rein. My training mantra:
think about turn. Picture in head. MOVE Slowly!!! Don't jerk reins up and over. Give horse plenty of time to feel, setup, and respond. (i.e., don't ask for a turn too close to the wall). As soon as horse shifts weight to acknowledge drop cues (otherwise you are nagging). Always show (tweak, big cues) then practice.
We worked on these cues intermittently for a week before I felt the light bulb truly went on and stayed lit. He took real well to the curb (prefers it over broken bits) so I didn't fight with bit issues at the same time.
As your horse's light bulb comes on, you first drop the tweaking with the direct rein and then you lower the rein further down the neck.
Always remember to hold the reins in your inside turn hand. Neck reining requires you to be comfortable moving reins back and forth when starting out. This provides the horse with a much greater cue (your whole body is engaged properly, rather than off balance).
I didn't try anything fancy at first like complete turns. I stuck our training sessions to 90 degree angles. They made the most impression and were the easiest to reward
We started neck reining in Spring 2012. Today, Sam can do a complete circle in both directions and will make minute course adjustments with mostly leg pressure and barely a lift of the reins.
I have been told that once the horse has been taught and gets it, you want to continue to drop the neck rein action and have the horse respond to seat and leg pressure primarily and neck secondary.
This may be due to the neck rein set is mostly in curb bits that need very little pressure to catch the horse's attention. It is "kinder" to cue off the leg and use the neck/bit only as necessary. Liken it to the difference between a gentleman putting his hand on a lady's back to direct her through the crowd and grabbing her arm.
I've ridden bitless before. My experience with those headstalls are they are better served as a direct rein approach. Although, that may be due mostly to the method of headstall construction and where the pressure points are.