Does the PBS system track individuals with multiple episodes of "lost" books? 

Yes!!  Absolutely!! The team definitely monitors "lost" book transactions.

They will contact a member, or even put their account on Hold until they verify the reason for all the "Lost" books.  I was even contacted once when I'd ordered 5 books from a single Sender (who never mailed them out but marked them as Mailed) when all 5 went lost on the same day.

Members with a habit of letting books get marked as "Lost" books can even have their accounts closed if there is no valid reason why they are not marking books as Received in a normal and regular fashion. 

This is good to hear...I sent one a couple of months ago (a wish list book) and the recipient never acknowledged receipt.  I sent an email reminder nothing.  Now I see their account is on hold.  I don't suppose the senders of the lost books ever get credit do they?