you're now realizing the trouble with a tall enclosure: that heat from the lamp has a long way to travel before it reaches the floor, and since heat rises, this makes for a tricky situation. how are you regulating the lamp? i don't think you mentioned it, or i'm blind. either way, your best bet is to run the lamp a bit warmer and see where that gets you. what's your ambient temp reading at the substrate?
EDIT: also are you running a CHE (ceramic heat emitter) or a regular heat bulb? and what wattage is the bulb?