I have attempted on multiple ocassions to drive my tapcon screws into the masonry block of my window sill ledges, but have been unsuccessful. Everytime I try to drive the screw in, I hit what seems like a piece of metal. My house is a 1950's colonial made with concrete/cement block and aluminum siding. I'm not sure if there is a piece of siding behind the block or if I'm hitting some other material used to "fill in" the block. Any advice on how to drive these screws in,so that I can finally hang my window boxes would be greatly appreciated.