Low stock report for tracked product attributes

I've just set up my first nopcommerce 1.7 shop.
I'm tracking the inventory for the products and using the low stock report. It's working fine expect for a t-shirt. The T-Shirt has product attributes (size) and the inventory is tracked by product attributes.
Nor can I define a minimum stock quantity neither do I see the t-shirt in the low stock report.

How do I get my t-shirt variants in the low stock report?

Thanks for any help.

Bettina Dober