Veritaserum works best upon the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it. Barty Crouch had been attacked before the potion was given to him and was still very groggy, otherwise he could have employed a range of measures against the Potion - he might have sealed his own throat and faked a declaration of innocence, transformed the Potion into something else before it touched his lips, or employed Occlumency against its effects.