Force stops (Settings > Apps > App info) on Android versions 3.0+ will block pushes intentionally. If you force stop an app, Android takes extra measures to ensure that the app isn't performing any other actions, so it blocks push notifications as well.
|To resume receiving push notifications on the device where the app was force stopped, simply relaunch the app.|
We do not have any other method to force push notifications to appear on a device when the app was forcibly stopped by the user.
If you close the app from the visual task manager by swiping, then Android will not block your pushes, and push notifications will display appropriately.