There are a lot of standardbreds in my area, and they generally make good mounts. However, the ones who did race are often difficult to train to canter, because all of their early training taught them that it is NOT ok to canter. Also, a lot of them are oblivious to leg aids, again that not being part of their usual training.
That being said, I don't have too much actual advice to give, lol. Some of the people around here never get their horse to canter consistently! I think maybe they are giving up too soon? I was riding a standardbred for a friend quite a few years ago who had the same problem. This may sound base, but I got him to canter by riding as if he were. In other words, got him trotting fast, then started rocking forward and back. (Not Too hard, don't want to throw off his balance completely) Uncomfortable and a bit disconcerting for the horse, within about 8 strides he 'broke gait' and started to canter. I immediately praised him, and he continued to canter the rest of the way around the field. It was the funniest, gawky canter I ever sat, but canter none the less, lol.