Managing stock on variations doesn’t work

I just installed Stock Manager and I’m trying to manage the stock of product variations. I select “Manage stock” and fill in a stock quantity after which I save. After saving the “Manage stock” is deselected again…. so I guess it doesn’t manage the stock then. But why?

Furthermore, I put the stock quantity to 1 and then in the store I can still add the product 4 times in my cart without any warning…. If I only have 1 item left in stock, the user shouldn’t be able to put 4 items in the cart….
So is the stock management somehow not working for my products, or this plugin just doesn’t check on whether the customer buys more than is in stock?