First, what’s under your Senses config? Are all affiliations (neutral, friend, enemy (everyone is probably neutral)) checked?
And then I’m gonna need to see a lot more Print String in there. How are you doing this without them. This looks way too clean to be functional 
I’d establish first that I can get it in a print string. Behavior tree brings its own potential issues.
You’ll want to check stimulus also, whether lost or gained, because it’s going to trigger for both. It’ll never stop having a target otherwise.