The use of rename(old,new) in awstats.pl on Windows is a problem since it only works on the filename not including directory/folder paths.
Trying to -update data files was resulting in a "Warning: Failed to rename" and the loss of the data file (e.g. a monthly summary).
use File::Copy; with move(...) instead of rename(...) in awstats.pl solved the problem.
This was with awstats 7.3.