I've had this happen before to some of my scenes too; I solved it by merging my scene into a fresh new file.Since you are using xref's this might not apply, but might worth a try. Also I would try a few things:1. binding your xref's into your scene and see how that works out (obviously not a solution, but could potentially help you figure out the problem.).2. remove your xref's one by one and try to render to see if that changes anything.
randomizing UVW mapping in RC does not work with Corona. It is also possible to disable instancing in RC, maybe that is the issue?
I dont know details how RC works, but you can determine it from the number of unique/instanced primitives - unique should be fairly low and isntanced high, if both are +- same, then there is no instancing going on
ok, so it seems RC decides not to instance it in the first case, that is the cause of the problem