finally i found the true problem, which depends on user’s network, sometime the ads display, sometime not
The display of Google ads has a lot to do with the network that customers visit. Google’s ads need to run JS scripts, and the execution efficiency of this script directly determines the display of ads. If the client’s network is too poor, the ad may not be displayed.
Google 广告的显示和客户访问的网络有很大的关系。Google 的广告是需要运行 JS 脚本的，这个脚本的执行效率直接决定了广告的显示。如果客户端的网络太差的话，广告是有可能显示不出来的。