Hidden lines are generated in Design Master using the standard CAD HIDE command. Unfortunately, this can end up hiding items you want to be visible behind a duct. The solution is to move the items up on the drawing so they are above the duct using the Move Entity Above Ductwork command.
Ribbon: DM HVAC->Drafting-> Move Entity Above Ductwork
Pulldown Menu: DM HVAC->Drafting Tools->Elevations->Move Entity Above Ductwork
There is also a Move Entity Below Ductwork command that you can use to move items below the ductwork if you want them to be hidden.
Ribbon: DM HVAC->Drafting-> Move Entity Below Ductwork
Pulldown Menu: DM HVAC->Drafting Tools->Elevations->Move Entity Below Ductwork