When working with sheet metal, it is possible to notice that flattening your component may hide or remove bodies. This is due to a limitation of the flatten tool, as the flat pattern is only meant to be used on a single sheet metal body.
Components that are not part of the selected body are hidden from the flat pattern view.
You will need to remove multi-body sheet metal parts, and create the component in one sheet metal piece if you wish to have the entirety of the component flattened into one flatten feature.
For further assistance, please contact our HawkSupport team at 877-266-4469(US) or 866-587-6803(Canada) and firstname.lastname@example.org.