Swift Haneke only caching images from the first three rows of UITableView

Question

In my TableViewCell I am using this method to retrieve URLS from my server, then I create a thumbnail image a video, finally inserting it into an UIImageView.

var URL: String? {
    didSet {

        if gotThumb == false {
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
                let videoUrl = NSURL(string: Somelink )

                let asset = AVAsset(URL: videoUrl!)
                let assetImgGenerate = AVAssetImageGenerator(asset: asset)
                assetImgGenerate.appliesPreferredTrackTransform = true
                let time = CMTimeMake(asset.duration.value / 3, asset.duration.timescale)
                dispatch_async(dispatch_get_main_queue(), {
                    if let cgImage = try? assetImgGenerate.copyCGImageAtTime(time, actualTime: nil) {

                        self.thumbImageView.hnk_setImage(UIImage(CGImage: cgImage), key: "\(self.thumbImageView.tag)", placeholder: nil, format: nil, success: { (image) -> () in
                            print("Got Thumb")

                            self.thumbImageView.image = image
                            self.gotThumb = true
                        })
                    }
                })
            })
        } else {

            print("Already have an image")
        }
        print(URL!)
    }
}

As my title states. Haneke is only caching the first three images that are created by the above method. What happens is that no matter how many ImageViews I have in my TableView they are all being populated with the first three images

I am quite stuck on how to resolve this issue.