If some child build job fails to generate any archived artifacts, there's no indication as to which child job it was, unless the 'optional' box is selected for all of them. This optional flag causes each to be logged. Without it, it silently fails with:
Build step 'Trigger/call builds on other projects' changed build result to SUCCESS
Unable to access upstream artifacts area. Does source project archive artifacts?
Build step 'Copy artifacts from another project' marked build as failure