sounds like the screws are rusted. you can use a drill and carefully dril the denter of the screw head until it is removed. with all the screw heads removed. use a rubber or leather mallet to tap the boars off of the headless screws. you can then use a pair of vise grip pliers to grasp the remaining part of the screw and remove it. it may hel to use a 50-50 mixture of liguid dish soap and water to make a lubercant and put a very small ammount on each screw, letting it soak for several minutes, just like using a penitrating oil on rusty metal.