I am working on that right now actuallg! Find the perfect bit for your horse first. Then,do a little tug on your outaide rein. Don't jerk it but just do a little tug. If yoyr hose doesn't respond, wait five seconds then do it again. Hope this helps!
Try different bits. It might be a little too soft for your horse. If you can't possibly get her head down no matter what you do then try a training aid like a martingale, draw reins, side reins, etc. just to build up your OTTB's top line and muscle so it's easier for her/him to put their head in a frame.
Sorry this is going to be long but I hope it helps! To teach a horse to work into a frame, my trainer has always told me to start at the walk. Bend your horses head to the left for 4 steps, then to the right for 4 steps, then back to the left etc. Once you are all comfortable with this go to 2 steps, and then try 1. Make sure that you are lot using all rein, and are using a lot of seat and leg to balance your horse while you do it. Once this all gets worked out go to the trot, and then from there try to canter:) it takes time and a lot of patience because your horse will most likely not understand immediatly. but good luck!
Try to bend his head a bit to left and right, not to much but that his neck gets more flexibel. Try riding him with the reins a bit longer but not that long that you don't have contact anymore, then "play" with your hands on the reins, like a kind of rytmh so he bends his head a little bit down. Do this very gently and don't pull the reins, give him sometimes a little bit room and then play again. I hope this will help you! I had a racer to and this worked for him, he was verry sensitive but fast and this 'slowed' him a bit down and he got a stronger and flexibel👍🏻
Hi!!! I ride an ottb also!!! He is the same way, I would talk it up with your trainer but my trainer had me do a lot of suppling exercises!! Also dressagetrainingonlin is a YouTube channel and has so many wonderful tips and tricks!!! Good luck!!😊