next_post_link showing only within subcategory

Hi,

I am trying to get the Next/Previous links to link to only posts in a category, and I have managed to do that successfully with next_post_link and the TRUE parameter. However, when I do this, it only links between posts within the same subcategory. I want it to be able to link between ALL posts in this category, not just the specific subcategory.

To be clearer:

I have a Video category. Inside the video category I have three subcategories: Sketches, Standup, Interviews.

If you click on a Sketches video and then click next, it will only go to other Sketches. I want it to be able to go to other Sketches, Standup, and Interview videos.

Thanks for any help!