category not selectable on edit post

I am having trouble with a category in the admin. It is 1 of 4 categories, and it is a parent category but has no children. When i edit any existing post or add a new post this category is disabled and I cannot check the checkbox. However, if I go to the main post listing page, and do a quick edit, I the checkbox is enabled for all posts. I probably just need to remove the category and add it again, but thought I’d ask here.