Collisions With Falling Images Not Working? HELP!

Ok, well I have the code to make an ImageView fall down the screen and then also the code for a collision to take place if they hit. Now my problems are:

a) How to make the image repeatably drop down at intervals
b) How to get the collision working to display a UIAlertView.

Ok the codes are:

Viewdidload set timer:

Code:

- (void)viewDidLoad {
	
	[[UIAccelerometer sharedAccelerometer] setUpdateInterval:1.0/30.0];
	[[UIAccelerometer sharedAccelerometer] setDelegate:self]; [super viewDidLoad];
	moveEnemyTimer = [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector (moveEnemy) userInfo:nil repeats:YES];
}

MoveEnemy Void:

Code:

- (void) moveEnemy {
	
	enemy.center = CGPointMake(enemy.center.x , enemy.center.y +10);
	
}

Check Collision Void:

Code:

- (void)checkcollision {
	
	if (CGRectIntersectsRect(enemy.frame, ball.frame)) {
		
		UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Really reset?" message:@"Do you really want to reset this game?" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:nil] autorelease];
		// optional - add more buttons:
		[alert addButtonWithTitle:@"Yes"];
		[alert show];
	}
}

Header File:

Code:

@interface Aliens_Go_BoomViewController : UIViewController <UIAccelerometerDelegate> {
	
	IBOutlet UIImageView *ball;
	IBOutlet UIImageView *enemy;
	
	
	NSTimer *moveEnemyTimer;
	
	float valueX;
	float valueY;

}

@property (nonatomic, retain) UIImageView *enemy;
@property (nonatomic, retain) UIImageView *ball;

All the ImageViews are linked up. But the AlertView just isn't displaying. Also how would I repeatedly drop the image every * seconds?

Thanks!