vga to dvi problem

hello! i have a mini display port adapter to hdmi and dvi! My screen has vga so i buy a vga female to dvi male adapter and connect them! But my mac dosent reconise my screen !Also i have 3rtd screen with hdmi and i dont have problem but when i connect the dvi port my signal is lost!(all adaptors and cable is new and working) PLEASE HELP!!!!