U need to be the boss and not give her the option to not canter … mabey even use a whip and spurs
Are you asking her to canter by placing your outside leg back and inside leg in front of the girth? I know a lot of people just ask for the canter by squeezing more and she may think that just means trot faster (which it does)
Don't be super tense and if she is starting to trot really fast then half halt and ask again.
Try a few raised poles in the arena, and as she hops over them she might canter afterwards and try to keep her going. This worked for my horse that hadn't cantered for about 4 years with her old owners :)
Could it be that shes not comfortable with the 'smaller' area?
Your best bet is to just try push her through it and as soon as you get 2 strides into the canter praise.