Magnolias in trouble?

I’m a complete noob. Apologies. But I’ve tried plant apps and they don’t help. I have 35 mini gem magnolias and some do better than others. I’ve had three randomly do this and die and I’m not sure why. This one has just started and I’m wondering if there is any saving it or what’s wrong with it? The ones along the back struggled one summer because of the big trees. Any advice appreciated. Located in Victoria Australia