navigation controller pushViewController not working

I'm creating an application that has 2 main view controllers at the moment. The app loads into the initial viewController, and clicking a button inside should bring up the second viewController. Here's what I have:

AppDelegate.h

#import <UIKit/UIKit.h>
#import "ViewController1.h"

@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@property (strong, nonatomic) ViewController1 *mainViewCtr;
@property (strong, nonatomic) UINavigationController *navigationController;
@end

AppDelegate.m

- (void)applicationDidFinishLaunching:(UIApplication *)application {
    _mainViewCtr = [[ViewController1 alloc] initWithNibName:@"mainViewCtr" bundle:nil];
    _navigationController = [[UINavigationController alloc] initWithRootViewController:_mainViewCtr];
    _window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    _window.rootViewController = _navigationController;
    _navigationController.delegate = self;
    _navigationController.navigationBarHidden = YES;
    [_window addSubview:_navigationController.view];
    [self.window makeKeyAndVisible];
}

and my button method inside viewcontroller1:

- (IBAction)SessionNickNameSubmit:(id)sender {
    ViewController2 *secondViewCtrl = [[ViewController2 alloc] initWithNibName:@"secondViewCtrl" bundle:nil];

    [self.navigationController pushViewController:secondViewCtrl animated:YES];
}

but when I click the button the view doesn't change. I tried debugging and the code is hit, but nothing happens.

am I missing a setting somewhere?

UPDATE

I've updated all viewController variable names:

instead of ViewController1/2 I'm using mainViewCtrl and secondViewCtrl

but still no use :(