See the users list thread for details. Two issues:

• the changelog tracking: hudson only know about the top level module,
  not the externals, when it builds the changelogs

• the SCM poller only polls the urls provided in the configuration for
  the job: this means that if you have url A with an external defined to
  url B, then any change to B will go unnoticed by Hudson (Hudson only
  sees changes in A)