这个问题可能是因为在使用 Android IMA SDK 预加载广告时未正确初始化播放器或未设置广告播放器的监听器。以下是一个示例代码，说明如何正确初始化并获得广告事件：

// 创建一个 ImaAdsLoader 实例
imaAdsLoader = ImaAdsLoader.Builder(activity)
        .setAdEventListener(adEventListener)
        .build()

// 创建一个 ImaAdsLoader.OnAdErrorListener 监听器
private val adEventListener = object : ImaAdsLoader.OnAdEventListener {
    override fun onAdEvent(adEvent: AdEvent) {
        // 处理广告事件
        when (adEvent.type) {
            AdEventType.LOADED -> {
              ...
            }
            AdEventType.STARTED -> {
              ...
            }
            AdEventType.PAUSED -> {
              ...
            }
            AdEventType.RESUMED -> {
              ...
            }
            AdEventType.COMPLETED -> {
              ...
            }
            AdEventType.CONTENT_PAUSE_REQUESTED -> {
              ...
            }
            AdEventType.CONTENT_RESUME_REQUESTED -> {
              ...
            }
            AdEventType.FIRST_QUARTILE -> {
              ...
            }
            AdEventType.SECOND_QUARTILE -> {
              ...
            }
            AdEventType.THIRD_QUARTILE -> {
              ...
            }
            AdEventType.IMPRESSION -> {
              ...
            }
            AdEventType.CLICKED -> {
              ...
            }
            AdEventType.ICON_TAPPED -> {
              ...
            }
            AdEventType.ALL_ADS_COMPLETED -> {
              ...
            }
        }
    }

    override fun onAdError(error: AdError) {
        // 处理广告错误
        ...
    }
}

// 将 AdsLoader 传递给 a CastVideoPlayer 对象
private fun setupPlayback() {
    ...
    castVideoPlayer.setAdsLoader(imaAdsLoader)
    ...
}