does a search in a mass update have to be the same transaction type as the records you are updating? I am trying to update sales orders based on a search against vendor invoice but am getting 0 results. If I run the search outside a mass update I get the results i am looking for.

Is one of your criteria MainLine=F? Mass updates will only pull header lines and can only update header lines. So if you're using that criteria, I could see where that would cause nothing to return.

I can't picture a process where you could use mass update to accomplish this