Ipad wont retain epub books when syncronizing

I'm using an iPad 2 on 9.2.1, iBooks version 3.4 and iTunes 12.1.2 for PC

When trying to syncronize books onto my iPad, they just disappear.

I have about 150 ePub books in my iTunes library, and they syncronize just fine onto my iPhone 4.

But when i try to load them onto my iPad, iTunes shows them being transferred, and if you watch the iBooks app on the iPad, you actually see them, as long as they are being loaded. But as soon as the loading circle finishes, the entry disappears, and the book is not to be found in iBooks anymore.

PDFs work just fine, and there is a number of ePub files that work, always the same ones, but just about 20, that are successfully transferred, even when deleting them inbetween.