doesn't work on selected posts page

I can successfully display secondary content for other pages via templates. I have tried to incorporate secondary content in my custom template for my posts main page, but it doesn’t work. In Settings > Reading > Posts Page I’ve selected a page to use as the main posts page. The secondary content I enter in this page doesn’t display as intended. Why is this? I’d rather not hardcode this content. Thoughts? Thanks!

http://wordpress.org/extend/plugins/secondary-html-content/