ion-content and ion-footer have different $scope

I believe ion-footer & ion-content creates new child scope which is Prototypically inerherit from current scope. Below ionic code will give you better illustration that how it works internally, the scope: true, is responsible for creating a new child scope.

Code

.directive('ionContent', [
  '$parse',
  '$timeout',
  '$ionicScrollDelegate',
  '$controller',
  '$ionicBind',
function($parse, $timeout, $ionicScrollDelegate, $controller, $ionicBind) {
  return {
    restrict: 'E',
    replace: true,
    transclude: true,
    require: '^?ionNavView',
    scope: true, //<-- this creates a prototypically inerherited scope
    template:
    '<div class="scroll-content">' +
      '<div class="scroll"></div>' +
    '</div>',

You need to use . annotation will fix your problem

Eg.

If you are using variable as primitive like

$scope.volume = 5

Then you need to use:

$scope.data = { 'volume' : 5}

Angular Prototypal Scope Inheritance