When you say "Lulu seems not to like the file"... what indication do you have of this? Simply it not being published in the iBookstore? There's one issue I know of that Apple's validation software won't accept which isn't highlighted by epubcheck, and that's the use of filenames inside the EPUB file (and for the EPUB file itself) which include spaces. If that's the case with the EPUB file itself, you can simply rename the file to remove the spaces from within the name. To check inside the EPUB file and, if necessary, amend the file names used inside:
- Make a copy of the EPUB file and rename the original (e.g. "filename-old.epub" - just in case things go wrong!), then rename the copy to change the file extension from .epub to .zip.
- Open the zip file using WinZip or a similar application and extract the files and subdirectories inside the ZIP file into a new, empty directory on your hard drive.
- Go through all the files you just extracted, and if any of them have spaces in the file name, replace those spaces with underscore characters.
- Open the (X)HTML files, the .opf file and the .ncx file in a plain text editor.
- Do a global "find and replace" to change any references to the file names with spaces to the amended file names with underscore characters.
- Save and close the amended files.
- Create a new zip file.
- Add the "mimetype" file to it first, with no compression and no encryption.
- Add the rest of the content using the default zip settings.
- Change the file extension on the new file from .zip to .epub.
You should now be able to resubmit the new EPUB file in place of the old one, and if the problem was indeed being caused by file names with spaces in them, it should now be accepted.