iPhone Pass push notifications not received if sent when phone is switched off

Question

I'm using Pushy to send Pass's to iPhones. The notifications are sending but if the user's phone is switched off when they are sent, they do not receive the notifications. When turned back on, there is still no notification. They are however, received if the phone is switched on when sent. I'm not sure if this is a local setting on the users phone, a backend issue with my Java, Pushy, or my pass files. Any ideas would be great thanks.