@AndyStewart  Hello, 

It is possible that the power line environment is not good at PA8010P's location.

You could try a different wall socket for that PA8010P.

Also switch the location for PA8010P with another power line unit and see if the issue occurs only on that location.

The performance of the Powerline device will be affected by various factors, you could follow this instruction and try to improve the powerline rate: 
https://www.tp-link.com/support/faq/409/                

@AndyStewart  Hello, 

Thank you for your update, WPA8630P KIT contains a WPA 8630P & aTL-PA8010P, they should not have problem talking with each other. 

Please try a factory reset on the PA8010P and plug the PA8010P and another working- fine powerline unit in the power strip (reduce the effect from powerline environment), pair them and double- check the powerline rate.

( reset via tpPLC Utility)  https://www.tp-link.com/support/faq/1612/ 

May I know when powerline rate is 11Mbps, does the real speed also very slow from that powerline unit? 

If powerline rate is still very slow, it is possible the issue is on the certain PA8010P and you could contact the local retailer or local support to check warranty for that unit.