| I agree with Aslan larger prey items can help in stimulating a food responce. What wattage globe are you using and how far from is it from the basking site? How active is the animal throughout the day?
Kirby i think sand is a really good substrate. For my digging box, i use washed river sand, the different sized particles give the sand more support and find if moist it can easily hold a burrow. | 
