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...?