Family Sharing of Mac App doesn't work

My partner bought a Mac App from the AppStore (Nitro Photo) and, as she is in my Family account, it was billed to me. When I open the AppStore on my Mac I see it at the top of My Apps. But when I install and launch the App on my machine, I am not licensed to use it. Why?

I did try the App a few months ago, for the one week trial period (which is why she bought it). Now, even though the Family has a license, when I try to use it on my machine, the App says my free trial has ended. The same thing happens if I launch the App on my userid on her machine.

Is this an AppStore limitation on Family subscriptions (e.g. should I have installed it with my userid?), or a problem with the implementation of the App? or something that I am doing wrong? Thanks for any advice...