To increase the humidity levels in an incubator you can:
* add a jar or tray of water
* add a wet towel
I believe that low humidity is caused by high temperatures which dry out the water trays in the incubator and help the humidity in the atmosphere evaporate.
You could also try to increase the humidity levels in the room in which the incubator is. You may also need to regulate the temperature in the room in which the incubator is in so that the temperature in the incubator doesn’t get too high.