Whenever I try to push a message to a registered device using the Push for iOS8 Bluemix dashboard, a message "The notification was sent." is displayed but my registered device never receives the notification.
According to the logs of my application, the device seems correctly registered to the Push for iOS8 service, and I'm also able to send push notifications to my device using https://github.com/nomad/houston (using the same certificate that I uploaded to the Push for iOS8 service, but converted to the .pem format).
My device is an iPad mini 4 running iOS 9.0.2.
Fix was provided by the bluemix team: the name of the certificate uploaded for the sandbox needed to have "sandbox" in its file name (example: my-certificate-sandbox.p12). That solved the issue.