I have a few underhammers and run into this problem some times. I take a small hammer and tap on the non-screw head side. Tapping the pin tighter to the frame. Eventually it seems to ‘catch’ and the screw (with my finger on the other side) will unthread. It may take you doing it several times but it will work eventually. It has for me and I’ve used no other method. Don’t give up after a couple tries.