Book Covers Not Loading

We are building a web application that loads a collection of books from library thing and displays their cover followed by some information about the book. The problem we are having is that some book covers aren't being retrieved, while most of the books load appropriately a few just point to 1x1 blank images. Is there a reason the API would incorrectly return the link to the images?

EDIT:

I've done some additional looking into it and figured out part of the problem. Originally the covers were loaded from the .cover field of books we had called from the API which returned a link to either an amazon or librarything image to a book cover. The amazon book covers worked just fine but the librarything covers were tiny resulting in either blurred or incredibly small images depending on the browser.

I set it up after that so that the covers loading from librarything would instead come from covers.librarything.com/devkey/(devkey)/medium/isbn/(isbn) rather than links like http://pics.librarything.com/picsizes/9c/b5/9cb55cc516a603d597933585741414641415... which is what the api was originally returning. This fixed a few of the books but it seems that some of the ISBN's are either incorrect or some of the books aren't in that database. Is there another way that I could be loading these covers to avoid this?

First off, can you clarify something. These covers you are fetching, are you fetching multiple covers for each work, one for each size? Or are you just fetching the largest sized cover for the work? (Or something else?)

It will also help if you will give specific URLs of works that are giving you these 1x1 covers.

I ask, because sometimes LT gets covers directly from Amazon. Amazon will return a 1x1 cover if a cover isn't found. LT has some code to catch this, but my reading is that it isn't 100%.

But another annoying problem I've been seeing lately is that Amazon is returning 1x1 pixels for only certain sizes of covers on certain works. Here's a couple of threads where that turned out to be the case:

http://www.librarything.com/topic/134301
http://www.librarything.com/topic/134364

I realized my original post was a bit vague and missed the steps I've taken looking into the issue. I am only loading a single size(I think?). The books loading from Amazon are the ones that gave me no trouble at all interestingly enough, it's the books that return covers from here at library thing that are causing me grief. Or are those covers also from Amazon?

For an example:
These have worked from the beginning: http://images.amazon.com/images/P/0787901318.01._SX50_SCLZZZZZZZ_.jpg
But these never seemed to work: http://pics.librarything.com/picsizes/9c/b5/9cb55cc516a603d597933585741414641415...
And these are hit and miss: http://covers.librarything.com/devkey/2455022613/medium/isbn/0787978086

Yeah, I can't say for sure why LT wouldn't be finding an image for that last one. If I go to that book via ISBN it seems to have a cover just fine. Even from Amazon, it appears to be in all the various sizes. So I have no idea why they're not returning.

Thanks anyway Bright. I'll be sure to report in about this if I figure it out.

It seems to me that your next step is to report it as a bug in the Bug Collectors group.