It is possible that registration to GCM will fail for various reason. Some of them are listed here: http://blog.pushbullet.com/2014/02/12/keeping-google-cloud-messaging-for-android-working-reliably-techincal-post/

It would be useful if Android code would contain a repetitive registration, so users won't need to repeat similar code snippet themselves: https://github.com/TadeasKriz/devconf2014-android/blob/master/src/main/java/com/tadeaskriz/devconf2014/MainActivity.java#L98-L115