looks like docbook5 requires clause-which-include-entity-file like
<!ENTITY % externalentity SYSTEM "external.ent" > %fexternalentity;
in *all* docbook files which use entity defined in it;
even if the main file have the line, those entities cannot be used
from included-files if the included-file do not have the clause
DocBook V5.0 The Transition Guide should have this information around
"Converting DocBook V4.x documents to DocBook V5.0"