Make sure both units are set to the same channel (not for DECT models).
It takes about between 10 and 30 seconds maximum each time, before a link is established and confirmed by the LED indicator.
Check if the batteries/battery pack of the Parent Unit is ok. Make sure the baby unit is connected via the AC power supply adapter to the mains (internal batteries are for power failure backup only).
You are out of range of the transmitted signal; e.g. there are to many walls and/or ceilings in the signal path. Just to verify the correct operating, try at a short distance (min 5 mtrs), in a totally different environment or even outdoors.