Blog and list/detail Navigation.

Got a really weird issue with navigation in the blog and in a list/detail type solution.

Everything is set up as usual in Perch with the post and archive pages listed as sub pages underneath the Blog page.

However the 'Blog' option on the navigation just won't stay selected when I click through to a post.

The list/detail part of the site is doing the same.

Mystified. It's something I've done happily on countless occasions but I have noticed that my site which I've recently updated to the latest version is doing the same.