Ted, instead of using the Putty knife, try a tack puller under the screw head, as they work well at applying more equal upper ward pull force, has always done the trick for me when small screw has stripped the thread in the plastic assy. boss of various devices.
One other trick that works well is if enough of the head of screw is exposed, (tighten) the chuck of a drill on the screw and use the drill to reverse it out with upward force.
Tack pullers are cheap and handy , found sometimes at the dollar store or any hardware store. Link shows tack puller. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
http://www.acehardware.com/product/index.jsp?productId=1288974