ASIN inactive after adding it to parent

We had an active listing under which we were selling a pack of 1 using FBA. We then created a new listing for a pack of 2, and another listing which became the parent. Under the parent listing we added two variations; one using the ASIN for pack of 1 (which was already active) and a pack of 2 which was the newly created listing. Since doing that the active listing is now showing as inactive out of stock although the available quantity is enough at the FBA centre and is showing in the manage inventory list.

When I edited the parent listing the unit quantity for pack of 1 was shown as 0 - I changed it to match the inventory to the no of units available at FBA which did activate the listing but the orders started coming through as FBM rather than FBA.

What am I doing wrong here?

Many thanks in advance for your help.