Make sure he has a good trot and is in balance before you ask, and bending correctly. Also try asking over a raised pole, with the pole raised on the inside
Im glad you posted this TTC having the same problem with my lad. He has come so far in his lunging since I got him but one rein I just cant get the correct canter lead. Driving me mad!
is it the left lead he picks up when going to the right? in my limited experience of our own horses it has always seemed they favour the left lead when they are getting established. Be interesting to find out if that is common with other horses too?