Seems that themes are different in the way that they handle menus. I usually use the Fusion theme on my Inner Quests site and I get a full drop down menu that reflects the parent/child page structure in the panel on the left side of the image below regardless of whether the custom menu I created has that structure.

With the Minimal Xpert theme I need to make sure that the menu panel reflects the drop down structure that I want. I also have to identify the menu I have created in the right panel as the ‘main nav’.

This simply means that some themes are more ‘fussy’ than others and may be the root of problems with the drop down appearing.