Mobotix app V1.4.2.0 no alarm iphone on lockscreen

I am using the Mobotox app V1.4.2.0 in combination with a Mobotix M24 at my gate.
The app on my iphone and my Ipad works very well, but there is one problem.
When someone is at the gate, the person is seen by the Mobotix with movement (VM) and the alarm rings at the app of the iphone and the ipad. But only when the screen is on I her the alarmsound.
When the screen is in locked, so the iphone or ipad screens are are black, than there is no alarm sound, only the message that there is somebody at the gate is on the screen as a message.
What can be the problem?
The iphone and ipad are both uptodate and the mesage-setup is good.