Try getting one of those waterfall feature things in place of the water dish. The little water noise it will make SHOULD draw them out. Also try leaving a radio on quietly beside the tank all day, shut off at night. The Changing noise of radio songs to commercials sure keep mine guessing.

Stimulation draws them out. Like changing the layout. I find noise works great for... ALL of my reptilian pets, garter snakes, lizard, king snake, even my scorpion(Not a reptile I know) comes out when I turn on the radio.

Both my Garters come out when I'm pouring water in the dish, they LOVE the sound, I;m going to see if I can get a waterfall thingy next time one is on sale.