Issue with folders

Hello!

I have my university work folders arranged in a particular way: My Documents > University > Year 3. Within the Year 3 folder are 6 more folders named after various modules, and within each of those are a number of powerpoint docs and ms word docs.

For one of these folders (named Biography) however I cannot create a new folder (no button to click at the top, no option in the drop down list, the shortcut does nothing).

If I create a new folder in the Year 3 folder and drag it into the Biography folder it disappears from sight but still shows up as an existing folder if I search for it using the search tool.

Any ideas why this might be happening? Every other folder works just fine, except 'Biography'. Thanks in advance.