A parry is different from a block. A parry is to divert the enemy weapon, not actually stop it. If everyone just blocked, weapons would break really quickly. If you think about it, it's not too unbelievable for a two-handed sword to smack a couched lance out of the way, especially at the M&B speeds of horses. But, you're saying that you thrust. This is EASILY parryable. Stop thrusting, start couching.