If you turn off the "Notify by voice" option under Notifications in the Nexx Home app do you get the normal text notifications just fine? If you are getting the normal text notifications this usually means your phone is killing the process that handles the voice notifications from running in the background causing voice notifications to either be delayed or not show up at all. The voice notifications require a bit more processing power than the normal notifications and sometimes the phone tries to kill the process running in the background to preserve battery life. 

A few other things to check as well - 

Make sure you are not logged in to a different phone using the same Nexx account email/password. If you need to access another user have them create their own account and share access using the Just Share feature instead.

When testing the notifications make sure you use the wall switch, remote control, or another phone that is logged in as a shared user. The notifications were designed to be received when the app is not active on the phone (like if you were working on something else). If you use the app on your phone to activate the door then you can easily see the door status and there really wouldn't be a need for notification.