Platform game

ive been trying to do this for ages. im just trying to make a platform game style movement but this code doesnt work :

onClipEvent (enterFrame) {
 if (_root.falling=true) {
  this._y += 10;
 }
 if (!this.hitTest(_root.floor_mc)) {
  _root.falling = true;
 } else {
  _root.falling = false;
 }
}

it is supposed to stop on the movieclip called floor_mc.
does anyone know why it just falls though the floor