Sub Menus Not Showing

Hello,

I am new to wordpress but have been a C# developer for over a decade. After creating categories I then went to Appearance and arranged them into heirarchical tree for my menus. My theme shows *ONLY* the top level menu nodes. Sub menus are NOT shown. Is this a limitation of the theme I am currently using or is it a wordpress limitation….or do I just need to be edjumakated on how to do it properly?

TIA
JB