Many (very many!) of my attempted matches are thwarted by having been merged on FS with a more current ID then deleted in FS. On reviewing the same persons in my RM7 from which my RM8 database was created, I find that the Person Tools fetches up the correct, updated, current record and successfully handles data exchange. I am surprised not to see others having posted this issue here already! I despair at my uniqueness. 
Some of the miss-matches cleared up upon recreating the data from GEDCOM but not all.