I used to have the same problem but fixed it by getting my canter while in a circle. The horse also had a hard time and it helped him a lot as well.
get correct bend and flexion, and put your outside leg a bit farther back that your inside leg. this makes you move your seat in the correct position
I would suggest you working out, especially working on your core stability since you need that to stabilize you in the saddle and angle your seat tge right way to properly ask for the right lead.
Lift your inside rein a bit sink into the saddle and push from your tailbone into the front of the saddle. They pick up the lead a lot better with that
you can also try asking for the canter in a corner or on a circle so your horse has a true bend
my horse will often stop too because he gets confused at what i’m asking for. i always make sure he is bent in and my outside leg is back, but it doesn’t seem to do the trick :/