BehaviourTree Boolean

Hi ive been struggling all day with something really simple.
I have a sequence in a behaviour tree and when a Boolean variable turns true I want it to execute a command that sends my guard back to the guard post. In short when my character dies I want the enemy to stop shooting me and go back to the guard post.

In order to set the variable in the tree I make a task, then cast to my player character bp, get the is dead Boolean. and then I cast to the ai controller and set the blackboard value as Boolean. however no matter what I do it does not change…i must have the cast to the ai controller wrong or something because even ticking the box myself does nothing.
Can anyone help me figure this out as its really starting to ■■■■ me off now lol and no one seems to help on the unreal forums. Its so simple and once I get this down I can do a lot more with it.

thanks