Because you are not initializing itemSearch using storyboard. You should initialize it using storyboard like-
NSString *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard_iPhone" bundle:nil]; //specify your storyboard name
xSearch *itemSearch = [storyBoard instantiateViewControllerWithIdentifier:@"xSearchStoryboardIdentifier"]; //You need to provide storyboard id in storyboard(Identity Inspector)
And then push it
[self.navigationController pushViewController:itemSearch animated:YES];