1. Make sure both units are set to the same channel (not for DECT models).

2. It takes between 10 and 30 seconds maximum each time, before a link is established and confirmed by the LED indicator. 

3. Check that the batteries/battery pack of the Parent Unit is ok. Make sure the baby unit is connected to the mains via the AC power supply adapter (internal batteries are only for backup in the event of a power failure). 

 4. You are out of the transmitted signal range; e.g. there are too many walls and/or ceilings in the signal path. Just to check it is operating correctly, try at a short distance (min 5 mtrs), in a completely different environment or even outdoors.