Maybe try moving it up very gradually?
That's what I was thinking I have slowed down on jumping with her and now thinking of doing some pleasure or even something including running because she loves to run.
At 28 the fact that she still has no issues jumping is pretty great, at this point her age may be catching up to her and she simply cannot jump higher.
Thanks for all the advice
She is 28 year old 15 hh mustang/ quarter horse
Build up her flatwork to as high as she can!
I was lucky to audit some of a Robin Hahn clinic this summer. I am not a jumper, but I learned that if you can get your horse to the highest level of dressage they're capable of, it is likely to increase their jumping ability to around 8 inches higher than before! Stronger muscles on their hind end and along their back and then more confident over bigger fences :)
Free lunging or sending her through a shoot are a good idea. Also having a feel with your leg all the way to the jump to let her know you'll be supporting her all the way through the jumping process.
It could also be that she has peaked where ever you are right now - how high are you jumping? How tall is she? What's her build? How old?
Maybe try free lunging? Build up her confidence without a rider and give her big rewards then try it in saddle.