@AnotherRod 

Good day.

Please refer to the following suggestions：

1. reset the not-working TL-PA717 by holding the pair button for 15s;
2. then put this one next to another TL-PA717 which had already paired into the network;

PS: if you do not mind, please also try the final suggestion：

1. connect the computer with the utility installed to each TL-PA717 one by one and change the network name to be TP-LINK: https://www.tp-link.com/support/faq/1616/

If still failed, may I know how long have you had the power-line adapters?

Did all of them work fine together before?

Thank you very much.