I am going to assume you are trying to read the epubs with the iBooks app?
If so, from where do you get your epub books? If they are from someplace like Barnes & Noble or Kobo (Borders), then they probably have Digital Rights Management (DRM) protection on them. Which means that only the reading device supported by that store can read the book. For example, you can only read Barnes & Noble epubs on the Nook or Nook app.
To further complicate things, libraries also use DRM. They use DRM made by Adobe, which is used with Adobe Digital Reader - which some devices can read. They can be read by various apps (BlueFire reader and Overdrive, to name two), but not the iBooks app.
All that to say, the only DRM-protected epubs that can be read by the iBooks app are e-books purchased from the App Store.
Although that doesn't help you read books on iBooks - I hope it helps to understand why. DRM is the reason for most e-book reading "errors"...
Marilyn