Objects in Outliner, not showing in Game or Sequencer Render


I’m new to this so I suspect I’m just missing something simple here. I have created a level with objects and lights. I see them in the sequencer view. They are all in the Outliner, but when I render out from Sequencer, or go into Play mode, I only see the BP_Sky_Sphere and foliage. I don’t see the Houdini Engine Heightfield terrain, created in Unreal Sphere Moon etc. What could account for these objects being present in the outliner, and view, but not in renders out of sequencer or play mode?