Frequent stirring will prevent this. Always cover the food with additional damp bedding. You may need to add moisture from time to time in addition to the food. Never let the bedding dry out. Sprinkle it lightly with water.
Fruit juice is even better.
If the bin is too dry, the food will stop rotting and the worms will not be able to eat it. Place your worms into the worm bin. Be sure to dump in all of the old vermicompost that came with your worms. This compost material contains lots of micro-organisms that your bin needs to help keep the wastes decomposing so the worms can eat it. The process is very slow.