the return status from GCM is "Failed"
Android Device which is Receiving push must be online, if not GCM will pool notifications till it reached a mark of 100, It will start clearing the pool if more notification arrives at their end
large amount of Android devices (NOT ALL) do not receive any push notification.
Did you check you have correct GCM ID with each device? If not The device will never receive a single notification. If your device have a valid GCM ID and have received some notification then it must be a problem in connecting with the device.
Also device must have Latest Play Services installed to get the notification on board