Hello everybody,
I opened a SAP support ticket for that problem.
And - shame on me - support told me that App Push of Android Enterprise Apps ins only supported for Samsung and LG devices. So it is normal for my Nexus that Apps don't get pushed.
And the second device which was indeed a Samsung device had been blocked through a firewall getting Apps from the App store.
So - everything is working as designed 
Sorry for the confusion.
Ernst