Show main-menu subitems

Problem: Subitems of main-menu don't appear (not even appearing in the resulting site's source code).

In the theme-settings of my Zen-Theme, I enabled to display the main-menu. In Structure->Blocks I disabled the main-menu. Result is an appearing main-menu, but subitems are not shown. When I disable the main-menu in the theme-settings and enable the main-menu in Structure->Blocks, the subitems appear. When I enable the main-menu in both ways, it is shown twice (but once without subitems).

I want to use the main-menu in the first way (enabling its display in the theme-settings). How can I manage to display subitems?

Thank you!