UIDynamics Gravity is not working

Question

I want to apply gravity to a button when I get a success response from the server. I am creating the UIKitDynamics elements like this:

-(void) setup
{
    _animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
    _gravity = [[UIGravityBehavior alloc] initWithItems:@[self.saveShareButton]];
    _collision = [[UICollisionBehavior alloc] initWithItems:@[self.saveShareButton]];
    _collision.translatesReferenceBoundsIntoBoundary = YES;

    _push = [[UIPushBehavior alloc] initWithItems:@[self.saveShareButton] mode:UIPushBehaviorModeInstantaneous];

    [_push setPushDirection:CGVectorMake(0, 1)];

    [_animator addBehavior:_collision];
    [_animator addBehavior:_gravity];
}

Here is the web service code

-

    (IBAction) saveStory
    {
        Story *story = [[Story alloc] init];
        story.title = self.titleTextField.text;
        story.abstract = self.abstractTextView.text;


        [[StoryHTTPClient sharedClient] createStory:story success:^(NSURLSessionDataTask *task, id responseObject) {

            // success

                [_animator addBehavior:_gravity];
                [_animator addBehavior:_push]; // apply push to that gravity will kick off


        } failure:^(NSURLSessionDataTask *task, NSError *error) {

            // failure

        }];
    }

I add the _gravity behavior inside the success block and then I apply a _push behavior to kick things off. But nothing happens. What am I doing wrong?