Menu-Block not showing up on random pages

I'm using the menu-block as a primary links sub-menu. For example, what I want is when I click "History" in my primary links bar, a block shows up on the left with "1960-1970", "1980+", etc. and when one of those is clicked, the block should still be there for the different sections of History.

I think I set it up fine as it works perfectly on the History pages and on the "Welcome" page and a few other nodes. But on ONE of my sections, it just doesn't want to work. Example, "Recruitment". I set it all up the same exact way, with the pages "Part 1" and "Part 2" having the parent "Recruitment", exactly like I have it set up for "History". The problem is, when you click "Recruitment" in primary links, the menu-block just won't show up, no matter what I do. If I go through the Administration and click on "Part 1" or "Part 2", then the menu-block will show up on those pages, but again, not if I go up to the main "Recruitment" page.

It just doesn't seem to make sense to me, I don't see what's wrong. Everything is the same from the two sections, yet the menu-block won't show up on the main page of one. What are some ideas to look into? This is really bugging me.