I am assuming that you are using the correct screwdriver for the screw (pentalope type) and that the screw spins in the hole, but wont come out.
No fun! If you haven't solved this yet, here is an idea/solution.
Try using a strong magnet on the tip of your screw driver/ on the screw directly to see if it will pull out - Maybe it wont work because of the black gasket material on the screen legs to stop ingress of liquid from the screw holes. If that didn't help, check out the next part.
Our favorite friend, Superglue! This will probably destroy your pentalope screwdriver and screw in the process So what we are going to do is glue the screwdriver to the screw to let us physically pull it out.
Add some super glue to the tip of your pentalope screwdriver, not to the screw its self. Now, don't add to much, we want a microscopic drop on your screwdriver tip. Not the side, but directly the tip. - If you do add to much, you just glued the screw into the phone - and that is not good at all as you can imagine.
Stick the screwdriver into the screw nice and strait and let it sit without moving it for a few min - maybe 5 min to be sure- I have had superglue take way to long to set - but that should be plenty of time. The screw should now be glued/connected to your screwdriver, so you should be able to gently pull out on the screw away from the phone with no problem.
You might be able to retrieve the screw and save the screw driver with Acitone (fingernail polish remover) if you wish, but no promises.