[Roger V]

Have had that happen a few times. Been successful with using a square-shanked screwdriver with an adjustable wrench on the shank. Need to apply all the downward pressure you can on the screwdriver while turning the wrench. Use a screwdriver that is not loose in the slot. Equally well is a straight-blade screwdriver bit socket with a small ratchet and again put downward pressure. Even a bit for a drill-driver would fit in a 1/4" drive socket.