You go in. You have it all worked out in your head. You go in. don't embarrass yourself, but you walk out feeling like you didn't quite do what it was you set out to do.
This is probably because you were rushing. You blazed through the scene, you were the fastest one! Congrats! Woohoo!
Auditions are not a race to be won. Take your time, relax your shoulders, and DON'T RUSH.
It's far better for the casting person to say, "Let's give that another go, but pick up the pace." Than to give you a polite, "Thanks," because they didn't see what you were capable of. If you rushed, they didn't see it, they didn't have a chance.
Look the other person in the eye, relax your shoulders, and hit it, nice and easy.