This issue will be addressed in the new upcoming version of Zooz switches, where the shape and size of the metal plaque will be altered to better accommodate the ground wire.
That being said, we've been able to get every switch we tried so far to hold the ground wire. It's important to have the wire straightened out if it was previously wrapped around the screw. And sometimes you may need to bend the tip to the right (in the direction of the screw) so when you insert it, it catches the thread easily - just like in the image below:
Every set up is different, but it's also important to insert the ground wire into the terminal rather than wrapping it around the screw. Give it a try and let us know if you have any questions!