Compost worms live in the surface layer of the worm farm. If you can’t see them, you can check on your worms by gently moving aside the surface layer - the worms may have burrowed just below to continue breaking down food.
If worms reside below the top layer, this may indicate that the conditions in the farm are not ideal. This is usually caused by overfeeding, or if the food has become too acidic. It can also occur in response to overpopulation of another insect, animal or invertebrate.
Remove old food scraps, reduce the addition of acidic foods and add some crushed eggshells or dolomite lime to balance out the acidity. If your worm farm is overpopulated with another insect, animal or invertebrate see "I have bugs (maggots/pot worms/mites/flies/slugs/snails/ants) in my worm farm. What should I do?".