When you add additional clients it doesn’t automatically create the folder for you so you need to add this yourself.
Let’s say you’re adding account 6131400001, once you’ve linked to Dropbox you would add a folder as follows:
\Apps\Quick File Receipts\6131400001_SOME_NAME_HERE\..
Obviously the SOME_NAME_HERE bit you can change to something more recognisable, the important bit is the folder needs to start with your QF account number.
Now dropping files into there should result in them getting pulled into the respective account.