When buying new screws at hardware stores you have to check what you are buying. It seems the screws you bought might be plated like your old ones but the actual screw could be made of brass (non-magnetic) or stainless steel (most grades are non-magnetic). I often build pickups with dummy poles made of brass when doing custom or stereo pickups. The plated brass looks like an actual polepiece but doesn’t direct the magnetic field to the strings. When buying parts for your pickups take a magnet to test the parts. For the pickup to work properly, the parts must attach to the magnet.