Odd... I don't see any definite pattern. The categories with lots of products in subcats show them, cats with only one product per subcat don't show the subcats, but some cats with up to 10 products in a subcat don't show subcats.

What is in your /includes/templates/your_template/templates/tpl_index_categories.php between these comments?
Code:
<!-- BOF: Display grid of available sub-categories, if any -->
 
<!-- EOF: Display grid of available sub-categories -->