0. Test Environment
Windows 8 Enterprise English 64bit
1. Why?
Windows can only show a limited number of Overlay Icons (15 total, 11 after what Windows uses).
Programs like Office Groove, Dropbox, Mozy, Carbonite, Kuaipan, TortoiseSVN, etc, will hijack a bunch of the 11 possible overlay icons.
2. Steps
You can see what overlays are set up, and change them in the registry here:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\ShellIconOverlayIdentifiers
So, you can re-sort them by rename these items. Just putting numbers in front of the ones you want to use and "z_"'s prefixed to the ones you don't need.
3. References
http://stackoverflow.com/questions/1057734/tortoisesvn-icons-not-showing-up-under-windows-7
http://www.cnblogs.com/Jinglecat/archive/2009/08/28/1555792.html