Okay, I too am not familiar with what PDD-Nos is but regardless of that from an outsiders perspective (just reading your post only) I personally would not be so quick to dismiss this potential friend and her mother just because you didn't get a response yet.
Many, many, many issues could have taken place to cause the lack of return phone call or not being able to speak to the Mother....such as:
*Mom might have left for work before child got on the bus, some people still have (what used to be called) latch-key kids?
*The child is only 9, so when asked to speak to the Mom maybe only Dad was home and the child didn't relay that information?
*The Mother might be UBER busy. Work a TON and just not had the time or immediate want to return your call? Her daughter might not be lacking in friends and play-dates and setting up *another* play-date for her child might not be what she wanted to do on her weekend? Who knows?
*The Dad might check the phone messages and not relay information to Mom OR not want to be the one to call you back?
*This child might only live with her Dad? Maybe Mom is out of town? Maybe the parents are (recently) separated and Dad is not good at or want to deal with play-dates? Seriously, Who Knows?
*The child might not want to speak to your daughter at school any more because she is only 9 and in her mind she has made the effort 2x and been denied both times?
*The child, again being 9, might not have told her Mom your last request for her to call you back?
People get busy girl. I know I do. You yourself had something 'come up' (your procedure) we can not always count on other people following through on what we *think* is the correct time line....ya know what I mean?
~Phew! That felt LOONG, LOL! Now with that being said: Always, Always follow your Mommy-Gut. If you do not want your daughter to be friends with this girl for whatever reason that is perfectly fine! Your lil' girl WILL make a friend...hopefully soon...don't beat yourself up about this, you really did all you could have done...if it's not meant to be, it's not meant to be and probably for a reason. Everything happens for a reason, right?