we have noticed that for some products in our store that have no options and inventory is set to "track without options", if we add an option to that product and set the inventory to "track with options", when we try to add some inventory combinations for that product (In stock: Edit -> Add combination), the popup where you can add combinations looks strange, is missing some fields - see picture.
So.... after seeing the same behavior on demo.cs.cart.com, i was convinced that there was a stupid and simple explanation.
I found out that the problem was from not ticking the Inventory checkbox on the option.
The problem is that this checkbox is not available when you create the option! Only after you save the option, you have to edit it and you can tick the Inventory checkbox.