I have a mare that canters great on way, but the other it takes a lot more ooomph to get her going and to stay on the correct lead. With the minimal information that you have given the best advice I can give you is really make sure YOU are asking the horse to canter the correct way, ie: make sure you are using correct body posture and you aren't in the least pulling back at all. It seems like a lot of people don't realize that they tend to pull back just a bit, especially when really trying to kick.
As for the leads I have always worked circles, small, medium,large and figure eights when wanting to get leads correctly. If your horse is having difficulty getting the correct leads you need to just keep consistency. When asking to lope if you get the incorrect lead drop down to a trot and use your rein and correct leg pressure and ask again and again until you get the correct lead. If you are having difficulty trying to lope to begin with I wouldn't focus too heavily on the leads until you get your lope down first.
You will get there, it just takes a lot of consistency and a lot of work!
good luck! :)