Interference can occur when the parent unit is out of range of the baby unit. Interference might also occur if there are too many walls or ceilings between the parent unit and the baby unit or when other 2.4 GHz wireless devices are switched on (e.g. microwave, Wi-Fi router).
Put the parent unit in a different place, closer to the baby unit, or switch off other wireless devices.
The rechargeable battery of the parent unit may need to be charged.