Multi Store Problem - Can't increase product quantity

[koistudio]

I have a multi store site, Actually are 3 sites, a Main Site and 2 Stores, which has 1 product each store; I add a product in one store then I go to the other store and add the other product (I use the Multi cart Addon) but when I go to the checkout and want to add more quantity only can add the product of the store I'm in, when I try to add more quantity of the produc of the other store, appears this message (see picture attached) "This product is no longer available"

 

Any idea how to fix this?

[TomanDesign]

I have the same problem, Any solution ?