Problem with compiling to ePub

I have a problem with compiling to ePub. I am trying to bring three short stories together for a box set. Whenever I drop and drag either one or multiple items over to the new project to produces an ePub document it will not pass validation on I am therefore unable to publish in this format.
I have tried the import a Scrivener project feature with the same resulting problem.
The Kindle .mobi compilation works fine with the same document.
Thanks in advance for any help you can offer.