Product Option with Inventory tracking problem

I have one product with 16 variants and product codes. For one variant (that I’ve found so far) the quantity available is 2, so I click ‘add to cart’ and it puts one in my cart. Then when I try to increase the number in my cart to 2, it gives me notices that the quantity I want isn’t available and that the quantity has been updated in my cart, BUT it just removes that product from my cart entirely! Other variants of this product work fine. I checked the db to make sure the product code, combination hash are unique and I don’t have any ideas of what to check. Any thoughts? Thanks! I’m on version 2.1.1