2-step Verification No Popup only hear the sound? The 2-step verification used to work. But today, when I sign in the account, my android device does not popup the verificaiton window at all. I only hear the notification sound, but no popup and no notification at all. This is just weird. 

All special actions I did after its last working time are (1) create a second user account on android mobile (2) this is dual-sim cards mobile. I removed my primary sim card.

I am not sure if the above two actions are the cause or not. But I do hear a notification sound, but with no popup window at all.

So What is the problem? and How can I do the 2-step verification now? 

Thanks a lot.
I found the solution. I have disabled some permission on Google play services. This is used to work well. BUT maybe due to google play services upgrade automatically or something else I don't know, it no longer works with some permissions disabled. After enabling all permissions to Google play services, it works again.