Make sure you have PUSH enabled for that extension in the Extension Settings for that extension.
If that doesn't work, try deleting the account from the Android App and sending a new Welcome Email.
If that also doesn't work, I have seen cases where deleting the GCM project and recreating it with a new API key, etc fixed this issue. If you do this remember to send a new Welcome Email again after deleting the account on the phone first.
Just in case you need the instructions of how to create a GCM Project have a look here:
http://www.3cx.com/docs/configuring-google-push/