Make sure your full backup is set to "Full, Using archive bit (reset archive bit)". If you happen to have selected a different type of full backup, the archive bit isn't changed.
You might also want to check a few files and watch the archive bit. On Windows, use the "attrib" command to list filed with their attributes. (i.e. " attrib c:\temp\*.* ") Make sure it's getting cleared after the full's, and watch if/when it gets set back on through the week.