Eamonn:
When you say "you upgraded a 3.3 viewer", do you mean that you had widgets working with the FlexViewer 3.3 compiled version and you just copied your config files from the 3.3 viewer to a newer FlexViewer 3.4 compiled version?
I ask because I have found that performing this type of upgrade often times does not work. I need to resort to manually modifying the new 3.4 widgets to match the older version widgets. This in itself is sometimes tricky as there could be changes to the config files that you also need to account for where the widget was enhanced or the config was changed to make the widget more user friendly.