You must make sure that your Digital Editions (ADE) is authorized with the same ID that was used to buy the books.
Most vendors just use the generic Adobe ID.
You can find your currently registered ID by ctrl-shift-I (probably cmd-shift-I on Mac) to ADE.
menu/Library/Authorize Computer on the older more reliable v1.7.2
If the registration isn't correct, ctrl-shift-D (cmd-shift-D on Mac) will deregister.
You then need to register with the correct ID.
Some vendors use Adobe DRM mechanism with their own vendor id/password.
I'm not quite sure what the use has to do to get that registration correct.