cura only opens first piece of model

Total newb
When I create a model in 123d design and save it as an .stl or .obj and try to open in cura, I only see the first component part of the model? When I open a model downloaded from thingiverse or the like in cura, I see the whole thing. Help???

Do you need to group select all your features and make them one model and then export it as an .stl?
I’m not familiar with 123d.

If you export your design from 123d, can you import it back into 123d and see if it’s complete?