Hi @Peter J,
I have had exactly the same experience with the wagons, it's really disappointing.
I want to run a rake of 21 wagons, but I can't seem to run more than 6 or 7 without the coupling being unreliable. @gedlee's post is interesting, but we shouldn't really have to be doing things like this to get them running well out of the box. I've swapped the Rapido couplings on mine for the Dapol dummy knuckles to close the gaps between the wagons and these usually sort some of the traditional coupling issues out. However it looks to be the design at fault in this case, the NEM pocket drops down on a small angled arm and I think this deflects upwards under tension and the wagons just uncouple. If the wagons were lighter this might not happen, but even a small rake quickly seems to overwhelm the coupler arms. Dapol do seem to prefer using a kinematic coupler, but I wonder if these would have been better with a more traditional NEM pocket attached to the wagon bogies.
I posted an exploded view of the wagons on the N Gauge Forum, so you can see how they are put together here - https://www.ngaugeforum.co.uk/SMFN/index.php?topic=43805.msg588274#msg588274
Hope you get your issues sorted.
Cheers, Mark.