A PBS member ordered 2 books from me. I checked that person's WL and realized I had a book on that list. I clicked on that title and only 1 person was wishing for that book (presumeably the person who ordered from me). I posted that book but it does not show up in my account as a WL hold. Why?

It is probably in the one hour buyer's remorse period.  It should show up after that. 

Did you make sure the ISBNs matched? If it's a different version they may not be wishing for that particular copy.

Well if you posted it by ISBN and not from the person's WL page it could be like Sherry said and you have a dif version than what they have WL.  If they had it on autorequest it would go into the 1 hr buyers remorse.  If it didn't, compare your book's ISBN and binding type.  They could be WL for large print or a dif binding-hardback when your's is paperback or vice versa.  You can always PM the person and let them know you have a dif binding.  I posted a WL book and had a HB version of another of that person's WL books.  I pm'd her that I had it in HB, she added it to her WL and I posted it right to her. I got to save on postage and she got another WL book, just in a different binding.

God bless people like you ALL that check a person's WL to see if you perhaps have MORE books you can send them.  I wish the people that had my WL books all had additional books to offer me!!!!! 

Pay It Forward ==> I'm going to start doing this now too, whenever I am notified to send out one of my books...