Actually, they work across rings. Unless you mean physically separated circuits, houses with one ring per floor are fine for powerline. HOWEVER, its a real YMMV situation. If the house is new with arc fault equipment, powerline will struggle.
I run AV600 from TP link. It serves my upstairs PC, TV and PS4. All receive link rates of 150 and actual download is 40mbps which is my internet cap. Transfer from my PC upstairs to NAS downstairs (which is via powerline, then Ethernet, router, back to NAS) is 80mpbs. Very stable.
I have known people not to be able to use powerline on separate floors however. my house is 20 years old.
Also, check your sockets. often the exposed core has oxidised and by stripping re-socketing (as it were) you...