You go to an audition (doesn't matter if it's for TV, film, commercial, or theatre) and when you're done reading, the people in the room say things like, "That was fantastic," or "Wow, that was the best reading we've seen in a long time," or maybe, "You blew us away." They're all smiles, and gushing with enthusiasm about your audition. You walked out stoked to the max. You nailed it - and there's no doubt they're going to offer you the part, right? Except they don't. In fact neither you or your agent hear anything from them at all.
So, what's going on? How could they have been so genuinely enthusiastic about your read, and yet - not a word? Not even to tell you that it was between you and another actor, and it was a really difficult choice, but they went with the other actor because he or she is repped by the same agent as the star, or whatever. And even though you'd be unhappy about not getting the gig, at least you'd know why. But nothing?! Not a word after all that excitement?
Now here's another side to that situation. You go to another audition, and you know you sucked at it. You blew your lines (on both takes), you were wearing the totally wrong clothes, and when they gave you an adjustment on the second take, you read it flat instead of subtle. And you book it!!! Okay, you're happy - but WTF?
Note: If you've been going out on enough auditions, you've experienced both these outcomes.
If you go back to that CD at a later date for a different audition, you don't want to ask the casting director why you didn't book. First off, you're probably reading for a different session director, who wasn't at your other audition. But even if it's the same CD or SD, they're busy and don't have time to try and remember your audition and what happened. Besides, you're putting them on the spot and they may feel uncomfortable telling you why you didn't book. A professional actor just doesn't bring it up. Be glad they are calling you back and let that old audition go.
Note: If your agent or manager has a good, personal relationship with the CD, they may be able to call them and get some feedback, but don't count on it.
It would be nice if casting was always logical - you give a great read, you get the job. You give a sucky read, you don't get it. However there are so many other reasons why we get the job, or don't get it, other than just how we read at the audition. Some of the (silly, but true) reasons you didn't book are: too old, too young, too tall, too short, too blond, too dark, and on and on - you get the idea. Put the past audition behind you, and look forward to kicking butt on the next one.