[stumped]

Did you copy the entire library folder and all subfolders
If so, how.
You have to copy into your local Dropbox folder then let Dropbox sync it.that will take a while if it is a large library or a slow connection. Wait for it to fully sync. It has to be a complete clone of source library, with all subfolders in same organisation.
Folder uploads via browser to dropbox.com are broken/ not supported. Dropbox blame browsers and have no plans to fix.
What do you see if you vsually compare what is in dropbox with what is in the source folder. Of or compare folder properties. The counts of files and subflowers folders should match so should the size