This has been an issue for a long time.
I believe the problem is that when the device goes to “sleep”, Web commands aren’t coming through.

What I found that helps SOMETIMES is when you call the device or send an sms which wakes the device up, and if you send the web command right then, it goes through most of the time.

I think there maybe no easy fix for this as if antitheft mobile keeps the device awake all the time to be able to receive Web commands it would just drain the device’s battery. On the other hand there are lots of various messaging apps that are able to receive messages even when the device is sleeping, so maybe there is a fix.
On the other hand the send commands expire way too soon on my.avast portal, I think it should keep trying for a while longer.

In any case I’m very disappointed that these essential commands don’t work and I’ve been paying premium for a year now or maybe longer.