eBook file protected error on Cocoon reader

Hi,

I hope someone here can help with a persistent problem!

My wife and I both have Cocoon eBook readers (model BK7021) and have joined our local library.  We have both created Adobe IDs and have successfully downloaded several books form the library to a desktop PC and a laptop, both running Windows 7.  We have Adobe Digital Editions 2.0 installed.

We can both open and read the books on the computers but when we copy them to the eBook readers we both get the same error 'eBook file protected'.

Can someone explain where we are going wrong?

Thanks

Roger