Your units are to AV1 standard, I believe,this only includes live-neutral comms and not live-earth and neutral-earth which AV2 supports in addition.
This makes your setup more vulnerable to "noisy" plug-in devices like chargers and power adaptors.
IF it is interference causing the failure then it must be severe to stop "pairing".
As for the cause:
One or both of your units failing
A poor connection within your mains wiring (there are reports of just loosening and tightening wiring making a big fifference)
A new, or newly faulty, device plugged in somewhere.
To investigate I suggest that you first confirm that your stable is on same phase and is going through your main house consumer unit.
The latter you can check by finding a breaker or fuse in your house consumer unit that turns off the stable.
Phase is best determined by visual inspection, if you know how!
IF you have 3 phase power delivered then likely the stable would be seperate phase, your electricity bill should show 3 phase charge if you have this.
It is possible if you are on 3 phase supply that changes elsewhere have knockedbout your setup.
I would also try the system again with everything in your house and stable that you can unplugged physically and if not switched off.
I only have single phase power supply to my house.
There is a fuse in the house labelled stables.
There is a circuit breaker out in stables with two more fuses.
Up to recently this has worked pretty well for about 6 years.
There have been no changes to installation recently apart from some new plug socket faces in main house. The home plugs have failed since then.
I have Three homeplugs installed at this time. A pair within the house working normally and the errant plug in stables that will not pair.
At one stage they all worked. One day the whole lot went down. I re-paired them all within main house but when brought out to stables one refuses to pair. I have swopped them around with same result.
Do you have real fuses, i.e. bits of wire that melt to protect the circuit?
Or MCBs, miniature circuit breakers that trip when there is excess current flow?
Fuses will not affect PLC devices MCBs do.
Thr new socket faceplates, any of them have built in USB sockets - these can be a source of interference?
Is your house wired with ring mains - will be if wired or rewired in last 70 years?
If so you could have a ring break, incurred during the socket replacement.
IF your main PLC unit is plugged in near one end of the ring close to consumer unit and the ring then gets broken at this socket then the PLC signals will now have to round the whole rest of the, broken, ring to the consumer unit before hoping onto the stables feed.Before it would just be a short hop.
If the ring is broken all power sockets will still work, so you might not be aware of it.
You could try plugging the main PLC unit in far away from its normal place, or even close to the consumer unit if you have a socket there - then see if the stables unit works ok.
Cartridge fuses.
No USB points in faceplates.
Wired with ring mains 30 years ago.
Don’t know how to diagnose a possible ring break if all sockets work. I did it myself and was pretty careful.
I will try repositioning the House Powerline and see if I can obtain pairing. There is no obvious degradation in performance between the two HPs in the house.
A way to diagnose a ring break is to remove one end of the ring temporarily from the consumer unit then check that all sockets still work. You have to do this twice, once for live wire then for neutral wire.
Alternatively with all power off and every device unplugged or switched off check with resistance meter for ring continuetes.
Unless you are completly happy and confident doing this dont attempt it.
Cartridge fuses will not affect PLC signals.
I will not try that. I can relocate the mother homeplug to another part of the house and connect it to wired LAN and see the result.
Although I would expect the stables HP to pair with the other one in main house albeit with no internet connection if the problem lay with mother HP.
And as I said previously the the two HPs in house work well together and they are on separate electrical rings.
Then again I wonder if it’s a paring procedure issue. I have so many of these units lying around unused. When using the pair button which unit should be pressed first?The mother unit?Does it make a difference as long as it is a working plug previously paired on the network? I said previously I had units paired within the stables perhaps they established their own network independent of the house and I have put those ones out of synch.
I think I have it. Something happened to disrupt my Powerline network some days ago. As a remedy I repairedA and B inside the house. I also paired C using a socket in the house. When bringing C out to the stables I mixed it up with D and E and fitted one of them. In the diagnostics C D and E got paired leading me to believe the problem lay between house and stables. The penny dropped eventually. Apologies for wasting your time.