have you been 'just trotting' all along?
I ask because it's not just that you trot, but how you trot. if you've been trotting along and you see no changes, then you can see that just trotting may or may not help, depending on how you are trotting.
the horse has to NOT be hollowing the back due to tension.
the horse has to NOT just be slopping along with their head hanging out in front , or way up in the air.
the horse has to trot in a way that they are engaging the stomach muscles to make themselves reach under the belly more, relax the neck , and even tuck under the pelvis some.
things like going over cavaletti poles, going up and down hills at a controlled pace , backing up hills, backing up and making many up and down transitions, WITHOUT being strung out , are what help build a topline.