I was planning on changing the flippers on my machine to original Gottlieb ones instead of the Williams it has. I can not seem to remove the bolt/screw that are in the picture. They is one at the top an another to the lower right of it. I can't get them to turn even a little. The screwdriver I have has the bit that fits into them not sure the name of the bit as it isn't a flat-head or a star.., but they refuse to turn.. What can I do?