They do love hidey holes especially ones they have to squeeze into, kinda like my cats.... Anyway, try bribeing them with frozen blood worms at dinner time. If they're new to the tank or if you've made a change to the tank recently, they probably just need time to adjust.
They grow very slowly so you have some time to decide if you're going to get a bigger tank or give them away. If you do decide to upgrade the tank, get the biggest one you can so you don't have to 'upgrade' again in a few years.