I have two UIImageViews moving like sprites on a superview. Each imageview moves properly by itself but when I put both imageviews on the superview at the same time, their individual movement becomes strangely restricted to two different areas of the screen. They will not touch even programmed to the same coordinates.

This is my movement code for the first imageView:

- (void)viewDidLoad {
    [super viewDidLoad];    
    pos = CGPointMake(14.0, 7.0);
    [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector(onTimer) userInfo:nil repeats:YES];
}
- (void) onTimer {
    pallone.center = CGPointMake(pallone.center.x+pos.x, pallone.center.y+pos.y);

    if(pallone.center.x > 320 || pallone.center.x < 0)
        pos.x = -pos.x;
    if(pallone.center.y > 480 || pallone.center.y < 0)
        pos.y = -pos.y;
}

and for the second imageview:

- (IBAction)spara{
    cos = CGPointMake(8.0, 4.0);
    [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector(inTimer) userInfo:nil repeats:YES];
}
- (void)inTimer{
    bomba.center = CGPointMake(bomba.center.x+pos.x, bomba.center.y+pos.y);

    if(bomba.center.x > 50 || bomba.center.x < 0)
        pos.x = -pos.x;
    if(bomba.center.y > 480 || bomba.center.y < 0)
        pos.y = -pos.y;
}

Why causes this strange behavior?

Thanks for your help. I am a newbie.