Hello  @Etchy 

Please make sure the powerline kit are used in the same powerline or under the same power meter and try a different room/ location for powerline adapters at first.
Try to eliminate the possible factors that affect the power line environment with the instruction below： https://www.tp-link.com/support/faq/409/

For ' looses connection' may I know does the House Led still light up when you move AV600 extender to a different room? Where AV600 worked fine before in the same location or its a new place for the powerline extender? Thanks.