All I can suggest (if you haven't tried italready) is to use a releasing agent - it usually comes in an aerosol can. (I've used a lubrcant/water repelling spray such as WD 40, with success with stubborn screws). Then leave for a number of hours or overnight. It also helps lightly tapping the screwdriver while inserted in the troublesome screwhead wth a small hammer. The more often you repeat the lube- hamering process the better.
I also assume the screws are tiny and difficult to reach. In that casee, use a toothpick wth a drop of the lube or release agent to apply to the screwhead. It ought to trickle down the shaft eventually, especially when applying the tapping.
Also note that it is not usually an instant cure but may take a day or two before whatever is locking up the screw to dissolve or loosen it's grip.