The display brightness may be set too low. Increase the display brightness on the parent unit using the control button in the middle.
Perhaps the lens of the baby unit is dirty. Clean the lens with a dry cloth.
The baby unit may be too far away from your baby. Make sure the baby unit is not further than 1.5 meters/ 5 feet away from your baby for optimal video quality. Closer than 1 meter /3.5 feet increases strangulation hazard.