Intermitant Glow Plugs

Hello,
My glow plugs don't come on like I believe they are supposed to in my 83' 300SD. Sometimes but, rarely do they come on when I turn the ignition on. I can turn it off and back on and the still won't after repeated attempts however when I try to start the car - it turns over usually doesn't start but, the next time I turn the ignition the glow plugs come on and the engine starts fine. Any idea what is causing this?
Thanks,
Tom Martin