Hi Karen,
8 months is a long time when you're in the throes of Google. Your problem might not revolve entirely around your navigation.
However in answer to your question - are they the right subpages?
Personally I'd do a card sorting exercise to determine which sub categories your users are expecting to see. You can normally back this data up with some keyword research to see which sub category type terms are generating a lot of search volumes.
For example if you've got a page called "English Literature 1800-1850" and more people are searching for "Romanticism period in English literature" then maybe you've got the wrong subpage.
It is a bit of work, especially joining up the keyword research with the card sorting but at least you'll know that you've got the right sub categories.
Hope that helps at least a little.