Start at the halt.
Use your ideal aid leg aid. The one you think the best trained horse on the planet would respond to. Almost nothing.
When your horse does nothing, or a very minimal something, give her one hellacious wallop behind your leg with a cross country bat.
Let her leap forward (if she doesn’t do something enthusiastic, you did not wallop her Hard enough) Praise her like mad and let her go for about 15 meters.
Halt again. Retest original aid. Repeat wallop if her response in not ENTHUSIASTIC.
DO NOT:
Repeat the leg aid.
Make the leg aid harder.
She CAN hear you, she has just determined that your leg aid is meaningless white noise.
When she makes an enthusiastic response to any gait from the ideal aid at halt, DO NOT wallop her. Allow her to continue in that gait. if she has shot past a working gait, great…bring her back to a working gait.
Then:
Slow the gait down a little. (10%)
Apply IDEAL leg aid ONCE.
If she does not make a minimum of a 30% change forward, WALLOP.
Allow forward for 10-15 meters, then repeat the above (slow down to a working gait, then ask with one aid for one response)
Keep doing min-transitions within the gaits, and real transitions between the gaits, throughout your work session.
The key here is to expect a response to every single leg aid, and to NEVER increase your ideal aid to a nagging one.
Your horse must understand how to avoid the wallop, and be impressed enough by it to be motivated to do so. Of it takes three wallops to get her to pay attention, so be it. Your safety lies in the horse being in front of our leg. You are not being mean. You are a coach with a lazy student who is blowing you off.