For some horses, getting one lead is often more difficult than the other. For ex-racehorses, the right lead is often a challenge. While working toward getting your leads better, the rider must ensure that the horse's back is sufficiently strong enough to begin canter work, specifically that the horse will be able to canter with a lifted back (not hollowed out) as his rider sits the canter strides.

Horses having difficulty with leads should be asked to canter from the trot, not the walk. The horse should be worked on a circle in the direction of difficulty, and the rider should ask for  the correct bend on the circle, so the horse maintains the arc of the circle through his body.

When the horse has given with his body and is correctly balanced and bent, the rider can open up the circle and take the horse out diagonally (or by leg yield). The rider shifts her inside hip forward and sweeps her outside leg. If the horse was correctly balanced and still holding the bend of the circle, he should move freely into a rolling canter. If he does not, take him back to the circle and through the bending exercise. Make sure you are not holding too much on the inside rein and that your primary contact is on the outside rein. The rider also needs to maintain her balance and not pitch forward as many amateur riders do when asking for the canter.

Keep practising the bending circle, opening it up to a larger one, then making a smaller one, then back to large. Keep your horse supple in his bend, and your canter lead will come.