For any screw that doesn't bend to your will easily it's essential to use the right size screwdriver. Make sure you're using one that fits snugly in the screw head and one with a good handle, maybe rubberised, that you can get a good grip on.

Then press hard - no, very hard - no, as hard as you possibly can - and a bit harder. If you feel the screwdriver might be beginning to slip in the screw head then you're not pressing hard enough. If it doe slip then the screw head will be damaged, making your job even harder.

Maybe the screw head is already damaged? Take a look at How to drive screws like a pro - there's a section at the end on rescuing damaged screws.