Please try this:
- run the Avast Mobile Security
- keep it in the foreground (so that you see the app on the screen)
- don't let the phone fall asleep
- send the command
If it arrives, it is clear - something is killing the app in the background. When it is terminated (by an optimizer or task killer), it cannot receive commands.
It it doesn't arrive, disconnect and reconnect your device to the Anti-Theft:
- open the app → tap Menu → Avast Account → Disconnect → Sign out
- select exactly the same method for login you use on my.avast.com (with Google, Facebook, or E-mail)
- follow on-screen instructions to link the app to your My Avast Account