My boy and I had this issue at first. He would trot, almost PACE really, before he ever really transitioned, and he would "linger" in that void between trot and canter for a stride or two. In the end, he just wasn't balanced and did not feel confident (and it didn't help that I anticipated it).
Try not to anticipate it. Be very, very clear, sit, and not only ask for canter, THINK canter. "Kiss" to her if you need to. If you have a dressage whip, maybe give her a little tap if she does not respond. Try to make the transitions clean, and if you can avoid it don't let her take of from an "extended trot" of sorts that they tend to get into. If she gets into that gait, bring her back down and organize her and try again. And praise, praise, praise. I have a video of me and Jack working on our canter from last week's lesson, if you'd like to see it - feel free to PM me.
Right now, we have only been cantering for a few weeks, and he's got clean transitions left most of the time, but he's difficult to the right.
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers