Julie,
A file or folder which begins with underscore (or a dot) is considered hidden throughout the Asciidoctor ecosystem and is thus ignored from processing (a common convention elsewhere as well). So the behavior you are observing is by design.
Since all of your docs are inside the folder that begins with an underscore, you might be able to work around it by setting the sourceDirectory to "_loadbuild/docs/adoc". The scanner should only be checking for the leading underscore on files and folders inside the source directory, not in the source directory path itself. (according to
https://github.com/asciidoctor/asciidoctor-ant/issues/54, this does not work as it should)
Cheers,
-Dan