I've noticed that the number of items in various layers in my wardrobe haven't gone up despite my having added to my wardrobe over time. Not sure if there's something I'm doing wrong or if it's on your end, but I've tried everything I can think of to trigger it.

To trigger layer achievements you have to go to the "Organize Items" link on the wardrobe page, then click "layer achievements" and select the layer you'd like to update from the dropdown then click the big button. Here is a handy link straight to the layer achievements page!