If I have a horse that doesn't know how to canter I ride with a crop for a couple of days. When you ask for a canter, tap with the crop if they don't do what you are asking. On another note, if you are unbalanced it is going to be uncomfortable for the horse to change gaits and that may be why they are reluctant to canter.
The other thing I do is go out on the trail with another horse. Have the other horse canter in front of you and then your horse will canter to keep up. I have had horses that hate working in the arena and won't canter in there. Even my little pony won't canter in the arena until I pop her with a crop. Out on the trail she is quite hot and wants to run all over the place.