DITA html 5 hierarchy different when using Tasks

In Framemaker 2015 I Have a Ditamap with various nested Concepts, Topics and References. When publishing to HTML 5, the hierarchy always puts Topics and References at the top level.

For example

Ditamap

     Concept 1

          Concept 2

     Concept 3

          Task 1

               Reference 1

     Concept 4

          Task 2

               Reference 2

Results in a HTML 5 output of:

Map Name

     Concept 1

          Concept 2

     Concept 3

     Task 1 (should be nested under Concept 3)

     Reference 1 (should be nested under Task 1)

     Concept 4

     Task 2 (should be nested under Concept 4)

     Reference 2 (should be nested under Task 2)

Where do I find the configuration or setting that will give me the required hierarchy?

Cheers

Peter