I've been a PBS member for about 5 years, and only oncee in all that time have I had a book that was shipped to me get lost in transit. However, lately, I keep having books that are shipped to me getting lost. Out of the last 20 orders I've requested, 2 books have been marked LIT and I have three more that are on the verge of their delivery windows and about to be considered LIT.

I've double checked that my address is correct - I haven't made any changes, moved, or anything else since I joined PBS, and I've always gotten books before without problem. So this can't be why my requests are going missing.

I'm really worried about this trend. I don't want to get kicked off PBS because books aren't getting to me. Has anyone ever experienced this problem and, if so, how did you solve it?

PBS will put folks on Hold and contact them.  They check how often you log in.  They will help if it is your post office.
 

You could also have run into a batch of members who marked the books mailed and then didn't mail them.  I'm sure that group is a minority out of the PBS population but just like damaged books or timed out requests some times come in batches, so could never mailed books.

Do any of them show as being mailed with DC or PBS postage that you can track?

I was thinking what Mary said, that it's possible that the books weren't shipped. 

When a book coming to me is close to the "lost" date, I send a PM saying that the book hasn't arrived, but that I'm watching for it, and that I know how to mark it received from the transaction archive it if arrives after the date.  I also ask them to let me know if they have any info on the book shipment that would help me to decide whether to order another copy or not.  A couple of times I've had a response that they had completely forgotten to mail it, and we worked things out from there.  One time (with a book being mailed to my son) the sender responded with shipping info, indicating the book had been delivered a week earlier.  It turns out it had been send via UPS rather than USPS, and my son discovered that the delivery man left it in a hidden place on the front porch (so it wouldn't get stolen.)  I was really glad I'd written. 

I still wait a couple of weeks after the "lost" date to reorder, since if the book was delayed by the Post Office (rather than mailed really late with no notice) I'll still owe a credit for the book even if I end up with two copies.

I had a problem a while ago with the post person (my old one had retired) delivering my books to the wrong addresses.  Some of my neighbors were very nice and brought me the misdelivered packages but I have a feeling a couple of my neighbors just didn't bother.  Do you have a new postperson or is your regular person on vacation/sick leave?

Three years ago my mail carrier went on vacation.  His first long one in a while.  The sub did not deliver a single media rate book for two weeks.  The PO has procedures to prevent this.  Not sure how the sub got around it.  But my regular carrier was quite pissed and apologetic about it.  

In addition, when my mail is held, everything is scanned as "delivered" when it is placed in the hold bin.  

I have started messaging the recipient the tracking number even when I don't use PBS tracking (so I'm not getting an instant credit).  Seems fewer people will fail to respond if they know the package is being tracked and PO says it was delivered.