Unfortunately, there is no information here to allow the user community to help. Can you please provide the version and build of IsoDraw as well as what specific CAD format you're using the version as well?
As with all things, version compatibility is the main hang-up. You may have an older version of IsoDraw that doesn't support a particular version of file. I know we internally had a problem a few years ago when our CAD group upgraded Creo without notifying us and we immediately hit problems opening files in IsoDraw.