Screen frame is very thin and will bend if you try to work it out from the center of the frame.
Also, if it has been a long time since the screens been removed, it is a good "jiggle" the screens while they are in the opening. For the homeowner this is most easily done from the inside of the house. Push up and sideways at all corners first. This breaks the crusty seal of dust that accumulates between the screen frame and the window. Once the screens can move around a bit you will have an easier time of lifting them out.
