The top of the material in your worm bin, being exposed to air, tends to dry out quicker than the rest of it.
And water tends to, from the effects of gravity, collect in the bottom of a bin.
To help keep moisture more consistent throughout the top and bottom of a worm bin and/or help the top of the material from drying out as quickly, some people utilize worm blankets.
Worm blankets, like fiber mats, bubble wrap, or damp newspapers, are placed over the top of the material in a worm bin to trap evaporating water from the worm bin and keep that moisture in the top layer.
They also help keep out light, and if you are using one you will see more worm activity on the surface level just underneath the blanket.
Just be sure that the blanket will not stop oxygen from reaching the wormies! Fiber mats are porous, but bubble wrap and damp newspapers shouldn't cover the entire top to make sure oxygen can still penetrate into the rest of the worm bin.