Sounds like you need to differentiate more between the empty folder and non-empty folder.
Ideas (you may need to apply multiple styles to get your point across):
non-empty folder: files sticking out of the folder or maybe the folder is 100% opacity or bulge in the folder or a full status bar
empty folder: folded open (empty) or opacity at 30%-50% or An "E" for empty or a none sign (a circle with cross through) or dust ball/tumbleweed or skinny folder (instead of bulging out) or an empty status bar
*Note: some of the ideas above are little ridiculous.
Regardless the solution you may need to explain the use a little bit, something quick and short.
Personally don't see it being a huge issue not know if a folder is full or not. But I guess it depends on the application. And I would try to explain this to the client, using references. Again not enough information to know what exactly is going on, need to know more about the UX, App, clients complaints, and what have you tired.