Books being de-authorized seemingly at random

I have downloaded three library books. All can be read on ADE on the computer. I transferred them to my Kobo eReader. Firstly when I tried to read book A it gave me an 'Oops, DRM error' message' and that I should sign in with my AdobeID. Books B & C could be read (I have read 360 pages of B and 735 pages of C).  I reconnected to the computer to retransfer. Now I can read book A but I get an 'Oops DRM error' with book B. If my device was not authorized I presumably would not be able to get a, b or c.