There's nothing else you can do, unless you used tracking.  If you did, you can report it to the post office, not that it will do much good.  Fortunately that doesn't happen often.  I think it's happened to me once.  From the Help Docs: 

A book I sent is lost in the mail!

Occasionally, if not marking books as received is noticed as that member's habit, a credit will show up months later.  That would be PBS doing housekeeping on inactive accounts. 

Even it that's the case Shelia, it's fairly uncommon at PBS.  There can be delays, especially during certain(busy) times  of the year...like Spring Break, Christmas, summer & back to school, etc.  Your receiver could be out of town or experiencing an unexpected life event.   PBS Admin does check on these if reported, but if they take action it can take awhile (they attempt to contact the member and give them ample time to reply).

PBS should get back to you by email in 48 hours.  With the understanding that is working days, not weekends.  A week is too long.  You should have heard from them.  You sent it in to the Feedback?  Bottom of the page under Company, then Contact Us, then Send Us Feedback.  (Live Help is volunteer members, not PBS staff.)

Can you check your spam filter or your club communications folder?

I have had 2 books on their way to me go lost in the mail and I have often wondered if the sender sent it ? or if it was mis-delivered and then not forwarded to me. I had a neighbor ( 2 blocks down ) get a letter from my son that was supposed to be for me . He was in Iraq and I was anxious to hear from him. And this neighbor finally looked me up in the phone book after holding my letter for a week. Duh ! I walked quickly down to get it. That is when I knew other people do not attend to things as quickly as I would. He could have put it out in the mailbox for the carrier or dropped the letter off at my house.

 Shelia other things happen as well and even here in the South we've had more than our share of Winter weather .; Hopefully that book will be marked received soon.

Did you check your Credit Registry to see if the credit came back without anyone contacting you? PBS may have contacted the member and they gave up the credit without any contact.

PBS may not get back to you immediately if they are working on finding out what's up with the other member. They usually give a member a little time to respond to them. They'll usually respond in 48 hours if it is just to answer your question, but if they need to contact another member and give time for them to respond, maybe some back and forth correspondence, it could take a week.

I did not use PBS DC but my receipt from the po tracking says it was delivered.

This won't ever earn a credit. Unless the other member is shown to be inactive, PBS won't grant the credit unless the sender uses PBS Credit Assurance. The payment for that service is what pays for the credit that is given.

If the credit is not in your Credit Registry and you do not hear from PBS in the first few days of next week, follow up with them.

PBS used to get back to people within 48 hours, but they hardly ever do anymore.  I know they've been working on the site a lot, so perhaps that may be the reason.  I don't know, I'm just guessing here.  I am always running into really weird scenarios where I need to contact them, and it often takes them quite a while to get back to me.  I'd contact them again after a week.  In the meantime, did you do any research on your own to see if the requester has an active account? 

Tangentially, difficulties that can arise in situations whereby a book was marked lost and credit has not been received for a book that had been marked mailed can be averted by using printable postage. For me the additional $0.55 fee for PBS printable postage is well worth it to avoid problems like Sheila's. The benefit of using PBS printable postage is that you get trade credit for the book as soon as you mark it mailed. This mitigates any issues that may arise regarding lost, damaged-in-the-mail, or unusual situations in so far as getting your credit. You will have a tracking record for having sent the book and will also have already received credit for sending it.
 

Sometimes books just get lost in the mail.

To put the actual loss into persepctive ....

I've mailed out 1600 books. Mostly without any DC. Out of those, 2 were lost in the mail and never marked received. If I had paid the fee for DC-only on all those books ... I would have spent $480 on JUST the PBS fee for DC, plus another $304 on the actual DC itself. When you balance the $784 I've saved myself against the roughly $5 in lost credits for lost books .... you can see why I choose to mail mostly without DC (although I do use it for multi-credit orders).

The OP has mailed out just under 400 books. And I don't know if she sometimes uses DC or never uses DC, but if she does/has, then that cost would have been roughly $196. Now she has one lost credit which she can replace in the book bazaar for slightly more than $2.

If people feel better using DC, then by all means, they absolutely should keep using it. But it doesn't really make financial sense, given the rarity of books actually going lost in the mail. I know DC fees also support the site, which is also good. But I do that other ways than DC.

My experience is that the Admin will take care of members when there's a dispute and the other member doesn't respond within 5 days.  But as others said, if they are trying to get in touch with the other member, particularly if they think the member might be dealing with a weather or medical issue, it can take longer.  It's  a small Admin Team, and sometimes things can get backed up, but again, from my experience, I'd say they'll take care of you.

Sara has made several very good points. I, too, have sent over 1500 books and have rarely had a problem with USPS delivery. So, is DC worth the additional cost?  

I got into the habit of using DC with it's accompanying $.55 fee because I also request a lot of books and want to be sure to have credits readily available instead of waiting until books were marked received. In addition, by using PBS printable postage you don't have to go to the post office for postage and mailing, nor have to pay for the service elsewhere through PayPal or some other service. For me, the convenience of using PBS printable postage continues to be a plus because I can print wrappers and mail easily and quickly. Plus, trade credit is applied as soon as books are marked mailed.  Getting instant credit is my primary motivation for using printable postage, followed by convenience.