要解决AVPlayerItem的isPlaybackBufferFull属性始终为false的问题，可以尝试下面的解决方法：

1. 确保播放器已经初始化并准备好播放：

let player = AVPlayer()
let playerItem = AVPlayerItem(url: URL(string: "your video URL")!)
player.replaceCurrentItem(with: playerItem)

2. 添加观察者以监听播放器缓冲状态的变化：

playerItem.addObserver(self, forKeyPath: "playbackBufferEmpty", options: .new, context: nil)
playerItem.addObserver(self, forKeyPath: "playbackBufferFull", options: .new, context: nil)
playerItem.addObserver(self, forKeyPath: "playbackLikelyToKeepUp", options: .new, context: nil)

3. 在观察者方法中检查播放器缓冲状态：

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "playbackBufferEmpty" {
        // 缓冲区为空
    } else if keyPath == "playbackBufferFull" {
        // 缓冲区已满
        if let playerItem = object as? AVPlayerItem {
            print("isPlaybackBufferFull: \(playerItem.isPlaybackBufferFull)")
        }
    } else if keyPath == "playbackLikelyToKeepUp" {
        // 缓冲区可以继续播放
    }
}

4. 在观察者方法中，确保在适当的时候移除观察者：

deinit {
    playerItem.removeObserver(self, forKeyPath: "playbackBufferEmpty")
    playerItem.removeObserver(self, forKeyPath: "playbackBufferFull")
    playerItem.removeObserver(self, forKeyPath: "playbackLikelyToKeepUp")
}

通过以上步骤，你将能够正确监听AVPlayerItem的缓冲状态，并获取正确的isPlaybackBufferFull属性值。