In my main navigation I have ‘Portfolio’ & ‘Photography’ links to plain html pages (not wordpress). These work fine on the index page, but when your viewing posts with a category, or an archive, these links don’t work.
On the single pages, the archive and category pages you are making the call for these pages from a different sub directory than what it is on the main blog page. The answer would be to hard code the entire link to these pages. On the photography link you have coded: