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