Alot of people have said some great stuff and had great points, but here's my two cents.
Its a combination of factors that can lead to terrible left leg kicks:
1. Flexability. If its not your dominant side chances are the muscles/tendons/ligaments are not as loose as those on your right. Because of this, when you throw the kick your body naturally counters the balance of the motion, but the tension on the tissues snaps them back involuntarily, throwing you off balance.
2. Technique. A lot of errors in technique i.e. not properly snapping the kick across, or not properly rotating the pelvis, or not properly bending the knee/flexing the leg muscles on the planting leg to counter the torsion of the kick
All in all technique comes over time, as well as flexabilty with liberal stretching will loosen the pelvis and legs.
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