Windows has a limit of 15 overlay icons. If there is more than this then the correct overlay icons may not show in explorer.

More info and how to fix here: https://www.dropboxforum.com/t5/Dropbox-installs-integrations/I-can-t-see-any-sync-icons-next-to-my-files-in-my-Dropbox-folder/td-p/366610