So someone said before it looked like her stifles were bothering her? Or just assumed, that since she was having problems picking up the canter, it is always stifle that causes that (I'd say that's too broad of a generalization).
--Wellsummer I gather that you would say, if not lame at trot...
I don't feel that if there's a cantering problem under saddle, that cantering ok on longe, in pasture, is a guarantee of anything particular, at least not for absolute sure.
It's possible it's simply that the horse has done very little cantering under saddle, and just was not used to it...