@phit I'll give that a go, thanks!
Edit: solved my issue, thanks! However I'm still curious as to what caused the issue in the first place...
If there are too many subcategories under a category, these categories are not displayed on the main page and on the categories page in the Admin panel. 10 categories under a category are displayed on the main page. More than 10 are not displayed.
You can increase that number in the settings of the category.