Unavailable routing?
 

Not sure if this is the right forum but in any event.... This is a purely theoretical exercise more to understand the vagaries of booking systems!

Does anyone know why a routing by two airlines that are otherwise in a JV would show up as completely unavailable when searching on fare sites?

So we have this routing- EZE - ATL - LHR.

DL flies EZE-ATL (DL110). VS flies ATL-LHR (VS104). However, if you try to force ITA Matrix to use this routing ("DL ATL VS"), it comes up as "no route found". It can't be due to MCT, as there's well over 10 hours to connect.

Is there some sort of obscure rule that prevents this particular routing or...?

Try "DL ATL DL", or just "ATL".

Fare rules. Just because a flight is a JV doesn't mean it can used for a fare by the partner airline. You need to remember that DL1234 ATL-MAN operated by VS is different from VS1234 ATL-MAN even though they're being operated by same plane. In essence it's two different flights. A lot of times the fare will require you use the flight coded as the airline selling you the fare.

Also, note that the JV with VS is restrictive. Due to needing AF/KLM/AZ permission to enter into JV with VS, the routes they can sell a combined fare are restricted.

Finally since this a a routing wholly between other countries, there can be legal issues involved selling a fare using another airline. While it connects in the US, the fare is really EZE-MAN. Thus both Brazil and UK have to ok the fare to be sold this way.

Basically there's numerous issues that could be causing a problem.

It will price just fine on DL.com as a through fare if you use the DL/VS version of the flight.

I think flyerCO is correct and it probably has to do with the DL/VS JV. Which I think is only for NA to UK. Which is why you can't find a VS code on the ATL/EZE flight.