Vuforia Studio: Header Button doesn't switch view

Hello everyone,

i am trying to switch to the next view using a button in the header widget, but that doesn't work.
It only works, when i open the Navigation menu, open any view and then go back to the first view and then press the button.
I am using this function :
$scope.nextView = function() {
$scope.app.params.text="call works";
$scope.app.fn.navigate("Explanation_mechanics");
}

and i call nextView() with the button in the header.
Also if the button is NOT in the header, but anywhere else in the canvas, everything works just fine.

I hope you understand what my problem is.
Thanks for helping!