When there are more than the default .yoda files of 400 produced, RivetMerge won't merge the ones after 400. This is somehow dictated by the --HerwigRun-number-of-jobs parameter, which sets the number of RunRivet branches and as a consequence also the number of RunRivet branch outputs to merge. It has to be investigated why exactly. However, if one is aware, one can still process all files by setting --HerwigRun-number-of-jobs to the correct value.