Could be anything really, is it a reasonably good quality adaptor or is it cheap crap..? I assume that normal lamps work in the socket, so it has power, right..? Is the lamp known to work..?
Having said that, 450W is a lot for a normal lampholder, but if it and adaptor both check out, it has to be the lamp
Try and get a normal mogul socket and wire it to a plug that you can just plug into a wall socket and see if that works.