iphone delaying notifications, then releasing them in a group when phone is unlocked

iphone is not sending notifications marked as immediate, immediately. I'm not sure what the status of the phone is when it starts collecting and holding the notifications. While that is happening no notifications from any app come through. It is not until I unlock the phone to use it that they all start to come in, one by one, with varying time stamps from up to 15 mins ago. What is causing the ios to hold the notifications and what can I do to ensure all of my immediate notification needs are delivered when they are due.