It's natural for a lesson horse to think he can get away with that kind of move, they will try and talk you out of your intended plan if it requires any extra exertion on their part, so in turn they take it out on your leg by exhausting it. When I see people who are having trouble maintaining a gait or tempo I make them count a few strides, usually three to five, and have them squeeze on a rythym, instructing the horse to continue as before. The funniest thing is the second I stop counting out loud the horse immediately begins breaking gait. So I have to remind them to keep kicking as consistently as they have to remind the hrose to keep trotting, viscous cycle. For that reason, next I get them to count out loud so I can hear and make sure, if they tell me "I'm doing it in my head" the horse will always tell me they really aren't. Anyways, long explanation short, maybe if you counted around the corners and turn and made sure you were consistently reapplying your leg every few strides your horses will notice. Many people think they are remembering to use their leg because they think about it, but putting it to the test is when you find out. I usually determine how many strides to count by having them squeeze once and seeing how many steps the horse trots before walking again, and then I have them do it one stride less. So if horse walks after 8 steps then they will kick on a count of seven. Or if it only goes three strides then they kick every two. I know you are cantering but the same exercise can be used at any gait. Don't know if this will help, but if I was trying to help you in person this is what I would have you try first.
Troubled TB ~"A thorn by any other name will prick just as deep." @-'--,---