Well, the back is supposed to attach without using any force or using the round button. It's supposed to 'snap' gently into place.
Furthermore, the back should not be able to detach when the dark slide is in.
If the back does not snap easily into being securely attached, and it falls off with the darkslide either in or out, then your back OR your camera needs to be serviced. My Hasselblad has started doing this with one of my backs lately, so I assume it's the back since it works well with a different back.
If you have other A12 backs you can try to load and unload and see if a certain back will repeat the problem.