Is the ground wire inserted into the terminal or is it wrapped around the screw? If it's wrapped around the screw, this may create an obstacle in the back of the upper wall plate screw hole and prevent you from cleanly inserting the upper screw.
Also, check that you're using the short wall plate screws that were included with your switch (rather than the standard long plate screws for regular toggle switches). Electronic switches like Z-Wave switches are built more tightly with all the electronics packed inside, so it's important to use the short screws.
You may have to adjust the length of the short screws included in the box in the rare situation that they're still too long to go all the way in. This may happen if the wall plate is particularly thin.
Lastly, using too much pressure and forcing the screws in may also strip the threads on the screws, making them unusable. Try not to use too much force when inserting the screws, but if it does happen, try using screws with a custom thickness or get in touch with us for help.