Cueing a canter in a line is not working. Help!

Hi! I'm getting ready for fair and the one event is with patterns. So i've been trying to get my mom's horse Todd to go from a trot in straight line to a canter. I'm cuing correctly because going around the ring I can get a beautiful canter out of him, but from the straight on he attempts but somehow he doesn't get into the gait... I don't know how to approach this. And he isn't young so he knows my cues. Help! How can I help my horse understand?

Perhaps you could describe the cues you are giving. Or could it be that you are leaning too far forward? Does he race into a faster and faster trot?
Are you bending him to the inside?

Okay, well I am beginner rider so bear with me. :)

At cone one I asked for trot from a walk then cone two I asked for canter with my outside leg since my right side was towards the fence so used my right (outside) leg to cue him by placing it slightly behind the girth and keeping the left inside leg steady to support his body. Before I cued though I half halted with my left inside hand/rein and saw his one ear turn to me. He does get distracted when riding especially since my other two were on the other side of the hill beyond some trees in the paddocks and we were at the flat training ground. But he was earlier listening to my cues for releasing pressure to cross step his legs ( not the right name for the movement) . Anyways, I kept my legs where they were and didn't have my "nub" spur it's like 1/2" long not even I guess but no spur or crop and I didn't go crazy kicking or anything just a slight lift of my ankle to see if I was putting any pressure on. Maybe I was forward but I couldn't say since mom wasn't with me watching my body positions. To say also, weather was hot and sweaty but yeah he'll canter around our ring just not at the cones and he's not affraid of them. Any ideas?

Oh yeah, he does quicken the trot and I can tell he gets agitated because he takes the bit and shakes his pretty little arab head around! He's cute but I don't know how to help him and me.

Sorry

And yes if he wouldn't change the gait then yes I would bring him on the inside because our arena is NOT large but enough size for me to work for dressage with my mentor.