During investigation of an unrelated issue (Thread 198387) it was discovered that the Android app installs with notifications turned off by default.
This was an unexpected recent change. And will definitely explain any recent inactive new Android app users.
To turn on again, hold your finger on the RHP icon, then choose "App Info" from the menu that is displayed.
On the App Info page, select Notifications, then enable them.
Note : Even if apps are enabled in the app itself (in "Preferences" page), without approving the notifications as described above, you may not receive any.
This will be updated in the near future so a prompt will ask to enable them after install, and then again when the preferences are changed.
edit : A new build is available now (5.1.1) which will prompt to enable notifications.