Hello   @Adsie 

Please try some tips below to manually pair the device

• try a different wall outlet and press pair button (on the bottom of the WPA4220 extender) for 1 sec again 
• try factory reset the powerline units and press pair again: 

Reset Wireless powerline unit: 
Press and hold the Reset button (or use a pin to press reset hole) until all LEDs turn off and start on again.  

1. Try to pair manually: please  put 2 powerline unit in the wall sockets, in the same room, and pair them again :
Paring Note:  press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again) When the Powerline Led or House Leds are on, the pairing process is done

FAQ: How to secure (pair) the Powerline Network?
 

2. Try to pair using tpPLC Utility 
Please factory reset your powerline unit by tpPLC Utility one by one:
https://www.tp-link.com/support/faq/1612/   

Refer to this instruction to pair the powerline units/ add an extra powerline unit by Powerline Key: 
https://www.tp-link.com/support/faq/1615/

If you have tried the above steps and  factory reset manually by Utility but Power led does not flash when press pair button on the bottom , that could be a faulty unit, you could reach retailer or local support for warranty. 

  @Adsie thank you for the letting us know the test result, feel free to let us know if the replacement unit works for you.