I've found why my project doesn't build as expected.
Our project layout has a root-level pom file (describing a particular module
configuration), that declares modules underneath it to be built.
Each of the modules are included in the directory structure by an svn:external
definition in the directory.
Each of the modules has a parent that is NOT the root-level pom, but that of a
corporate pom (this is a common m2 build structure).
When the project is examined by hudson, it reports that the root-level pom has
no downstream projects (and therefore when the build is started from a detected
change, very little happens).
I can't add a dependency between a module and the root-pom because there are
many root-level poms with different build configurations (and there is no such
dependency). I think hudson needs to add these child modules as build dependencies.