Windows explorer’s overlay icon is the small indicator at the bottom left of an icon that some applications use to indicate the status of files and folders. It looks like this:
The thing is Windows (Windows 7 to my knowledge) seems to have a limitation of 11 (or 15?) of these indicators being used by all applications. So if your system has too many of them, some will not show up. For example, I installed TortoiseSVN, TFS, SkyDrive, and Dropbox on my Windows 7, and Dropbox indicators got disappeared. The only workaround I know so far is delete some indicators that use don’t use to leave space for others.
More info on this http://productforums.google.com/forum/#!topic/drive/_zN32ac5n18.