I agree with AP - and will say it can take quite some time before they catch on to what is wanted.
I will also change from the normal snaffle, which, as racehorses they have been trained to grab a hold of, to a Myler snaffle which gives them a different feel in their mouth.
When you want her to trot a bit more forward although you should keep your leg on her sides, ask by rising higher so her stride lengthens to fit you. When you want her to slow rise slower and lower.