Both the parent unit and baby unit come with a charger. Check to see if the charger is at fault by swapping the baby unit charger with the parent unit charger.
Issue travelling with charger
Did the issue 'travel' with one of the chargers? This indicates that the charger is at fault. You can buy a new charger at philips.com/parts-and-accesories.
Issue not at the charger
If the issue persists and does not 'travel' with one of the chargers, there might be a technical defect with one of the units. Please contact us for further assistance.