I created a "Science" category, which shows up at the bottom of the add content page. But I also created a "History" category and that doesn't show up. Any ideas?
The best I can interpret your question:
A "category" in this context is typically a Taxonomy. If you see the "Science" category when creating a node, that would mean there is a "term reference" field added to that content type.
If you do not see the "History" taxonomy options when adding new content, I would assume there is no term reference field on that content type to this taxonomy.
Add a new "Term Reference" field to this content type. Ensuring it is associated with the "History" vocabulary. Then when creating a new node of this type, you should see this field.
If this field is added to this content type, then I would double check your "form display" settings on the content type to ensure this field is available.