Iphone 4s screw not stripped but wont come out

I have an iphone 4s which has alot of scratches on the back. This is the first time i am trying to remove the back of the phone but i can not seem to get the left screw out. The right 5 star screw comes out no problem. But the left one is stuck. Both screws are 5 star screws. I am using a pentalobe screwdriver.

Any ideas on how to get it out? I am still under warrenty so will apple be willing to get the screw out so i can replace the back glass?