On Sun, 3 May 2020 15:55:58 +0100 (GMT+01:00), Jimk wrote:

Any way to check that without taking them out again?

One of the cheapo ethernet cable testers will. Around a fiver from
CPC/eBay/Amazon etc(*). Or just loop back each pair (ie one wire
connected to the other wire of the same pair) and check for
continuity at the other end. In theory with the kit powered off and
network cable plugged in there ought to be continity through the
isolating transmormers in each pair. Never tried that so don't know
what to expect.

You mention a coupler and changing that made difference. Are the
plugs and coupler making proper contact on all 8 wires? No iffy plug
end?

Not quite sure of the topography. Is this cable effectively an up
link from the switch that the Pi's are connected to? What is attached
to the upstream end of this uplink cable?

(*) Simple but basic for about £30 you can get rather more
sophisticated tester that shows which end, or how far away, a cable
fault is and any wiring errors. Search SC8108 loads about also can
have up to 8 remote dongles that are numbered that shows on the
tester for cable identification.

--
Cheers
Dave.