Not getting notifications from 3rd party application when the phone is locked.

I am not getting notifications from third party applications such as whatsapp, instagram, Snapchat, telegram etc when my phone is locked. As soon as I am unlocking my iPhone and going into the application I am getting the notifications. I did all the troubleshooting, took assistance from Apple support as well but the problem is still there. Can anyone share his/her experience who in the past or currently facing this issue. If anyone has faced this issue in the past, kindly share what they did to resolve the issue.