If your home wiring does not use ground (like older homes), the PLA will not be able to reach full connection speed. Devices like the PLA5456 use all three electrical outlet prongs (neutral, line and ground) to reach the max speed.
Install the Zyxel Utility for the HomePlug device.If the devices are still not syncing please try resetting the adapter(s) back to factory default, it this does not work please try the following: Also, try plugging the devices on the same electrical outlets if possible to rule out line conditions being the problem. This is one of the biggest reasons why you don't see the devices synching, they are in standby mode. Please make sure you have a device connected to the Ethernet and powered ON so it links up with the HomePlug appliance.
Zyxel HomePlug AV devices have a standby mode which will put the devices to sleep if there is no activity on the Ethernet port. Use the LEDs on the adapter to troubleshoot the problem. *Multiple revisions released (ex: PLA400v1, PLA400v2, etc.)