I recently requested a book and today got an email the member is unable to mail. Should I expect an explanation as to why not? I was really excited to find this book available and eagerly awaiting it as it is a book I have been seeking for some time. Seems that there should be more than a simple is unable to mail this book somehow. Especially since it had been accepted and had a will mail by this date showing and everything and then suddenly poof, nothing.

These things happen for various reasons.  I believe if you check your Club Communications on the Main Page of your account there might be an explanation in a Cannot Mail message.  I'm not sure if listing a reason is mandatory but there's a good chance there is an explanation there.

Things happen sometimes.  It could be that they accepted the request and then couldn't find the book.  I ran a deal one time and absolutely could not find one of the books that was ordered.  I had to PM the person and have them cancel so the system wouldn't reorder it automatically for them from someone who wasn't offering a deal.  (I found the book months later in my TBR pile-where I looked several times and never saw it there). 

One time I posted a WL book and it was accepted right away by the 1st person.  I put the book on the kitchen table to remind to mail it the next day.  I set it down next to a box of baby wipes not noticing that one wipe was hanging over the side under the lid. (white whipe/white containor) The book spent the night touching that wipe and absorbing all the liquid.  I had to cancel the transaction because the book was ruined. 

If the book is available in PBS it's already reordered it for you from someone else.  If not than you can put it on your WL and be the 1st in line.

actually, if you got an email stating that the book cannot be mailed IF there is a reason stated it will say so in that email...