The Walker I used to ride NEVER trotted, but I could get him to canter, I just had to be careful not to let him pace faster (because he would if he was feeling particularly lazy). But to push him into it, I usually had to do it on a turn and rock like I was riding a canter, and we would fall into it without to much trouble. I'd seen him canter on his own in the pasture, so with him it was really just trying to move his body into the motions and he would just pick it up on his own.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.