Are you talking about Merges or Inheritance features?
Check the option for default_ext_ref_scope; if that is set to none, then you can't use external references. It could be some other configuration options, but check for that one first.
Also, has your license configuration changed? Some external reference features require AAX, so if you're not picking up that license, that could also be the problem.
David R. Martin II
Senior CAD Application Specialist