iTunes won't organise pdfs properly

I have some audiobooks with pdf accompaniments. When I try to sort them into the same album in iTunes, sometimes they do so without delay, other times I have to import something else before it will be sorted into the correct album, and sometimes they don't even get sorted at all. It seems to be totally random which get sorted and which don't - all of the ID3 tagging seems to be correct and the same. Does anyone know why iTunes seems to refuse to sort these pdfs?

Open iTunes and right click on the PDF and make sure the album/artist info on the PDF matches the album.

That's what I meant by ID3 tags (maybe my terminology was incorrect?) - I've checked all of them and they all match up, I've made all of them into a compilation, I've tried changing pretty much everything..