Asking from the corner doesn't help the horse. You probably realize that we ask the horse to do a slight haunches in when we cue for the canter. Ask your instructor to teach you to do this bc you can ride your horse at the walk this way and therefore, practice it. Horses will canter or gallop when they think they can be balanced. If the footing is off, or uneven, horses naturally drop back to the trot bc it is more secure, and they do not like falling or becoming cast, as in a stall. Their stiff backs make it difficult to get up again, and you see this with aged horses who are quite frightened if they have trouble getting up after rolling, for instance.
Anyway, at the corner of a rectangular ring the horse is slightly off balance and would prefer to trot it. If you ask even one stride away from the corner he would probably take your cue to canter.
Julie Goodnight has a good program on canter cues, and a new video that I think will help you. "Canter Master" Horse Master DVD with Julie Goodnight
I have been watching her program for a few years, but I remember reading her articles in "Practical Horseman" some 20 years ago. She has the demeanor around horses that I try to emulate, since I'm a little bit of a hothead, unfortunately! **Corporal looks down, embarassed**