Drag and drop image in landscape doesn't work

I have the following code to drag and drop one image onto another.

import Foundation
import UIKit

class DragImg: UIImageView {

    var originalPosition: CGPoint!
    var dropTarget: UIView?

    override init(frame: CGRect) {
        super.init(frame: frame)
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
        originalPosition = self.center
    }

    override func touchesMoved(touches: Set<UITouch>, withEvent event: UIEvent?) {
        if let touch = touches.first {
            let position = touch.locationInView(self.superview)
            self.center = CGPointMake(position.x, position.y)
        }
    }

    override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {

        if let touch = touches.first, let target = dropTarget {
            let position = touch.locationInView(self.superview)
            if CGRectContainsPoint(target.frame, position) {
                let notif = NSNotification(name: "onTargetDropped", object: nil)
                NSNotificationCenter.defaultCenter().postNotification(notif)
            }
        }
        self.center = originalPosition
    }   
}

This works fine in portrait mode, but if I change to landscape mode and try to drag and drop the image, the system doesn't recognize the image as being above the target image. Can you explain why and point to possible solutions?