If you could post a few burn logs, both from the +R SL that won’t play as well as the +R DL media that do play, it’d help a great deal.
You didn’t specify any particular app (I think you’ve mentioned platinum in the past ?), burn engine, etc., etc.
If it’s VSO…
The default location is:
C:\Documents and Settings[I]Administrator Name[/I]\My Documents\DVDFab\Log