What wattage bulb are you using at the moment??
Im using a 75 watt at the moment due to the warmer weather and i use a 100w in the winter.
Best thing to do is try a few different watts til you get the temp to around 110, i make sure it cant go over 115 as it could end up frying the poor sods
One of mine is a lazy ass too......he wont go hunting often unless its a wax worm which he will run a mile for lol