Apologies for getting kate back to you.
Could you please share sime more details like application logs at the time of Push notification send. Also please share the push logs which you can find under Unified Notification > Logs section . Please share these details, it will help us to debg the root cause of this issue.
Also, please let us know which sample plugin you are using and from where you are sinding the push notification.
P.S If you have not enable push logs, then please enable it through Settings section under App Manager.