There's a lot of missing context here so don't be shy in giving more details than you think might be needed. For example:
How was the subcollection created?
Was there custom code added to support that?
Is there a link the result can be viewed? Seeing it live can be helpful.
I jump on these forums to help and share some insights. Not looking to be hired, and not looking for work. Don't hand out staff invites or give admin password to forum members unless absolutely needed. In most cases the help you need can be handled without that. ★ http://freakdesign.com.au ★
I created collections to nest under my Accessories category. I moved the subcollections to nest under the Accessories tab. When you click on the subcategories it says "no products found." If you click on "sort by all" then you can see the collection. Thank you!