I have a question about what happens when I can't find a book when I search by its ISBN.  Specifically, I'm searching for books that I see are available on amazon.co.uk. 

When I try to put several books from there on my PBS wish list, the system tells me it cannot find the book.

I seem to run into this only when I'm searching for foreign books. 

Is there a way I can still get the book on my wish list?

When I search for a book I usually use the advance search option, and look it up by title and author name.  The only time I use the ISBN is when I'm posting books.

The Advanced Search option works better for more ISBNs than the regular Search option also.  I've had regular search not find a particular ISBN I was seeking, but Advanced Search found it.

Foreign ISBNs do seem to be hit or miss. Some are in the PBS database and some aren't.

Be sure you are searching all books and not just posted ones.  (And make sure you aren't just searching recent books.)