If you can't see them, you probably have them housed correctly. H. gigas are known for digging deep burrows, so you probably won't see them more than once or twice a year, except for maybe the tips of the legs sticking out of the hole. Make sure the substrate is damp, and that they have access to a water dish, and drop a cricket down or near the hole two or three times a week. As long as the crickets keep disappearing, you have nothing to worry about.