Unfortunately this is a current limitation of our integration with Dropbox, we only detect files that are dropped directly in the specified folder when setting up the zap. So if a new upload creates a folder and puts a file in it, the zap won't be triggered.
Right now, you can only watch for new files in a Dropbox folder if the folder has less than 4000 items in it. You'll receive a hard-error from Dropbox (and the Zap will not work) as soon as you go over this limit. We're working on increasing this though.
This error commonly occurs when uploading very large files. Unfortunately, our Dropbox integration cannot handle files over 100 MB, so you'll need to try and reduce your file size or move them over to Dropbox manually.
The Dropbox app requires file objects to be passed along during a Zap, and will not download files from a URL. One may consider using a Gmail 'Send Email' action as you can provide a public URL and create a file object that Dropbox can use.
Although other data for the new file is retrieved when triggered, the file itself is not pulled to our platform when its size is above 100 MB. This will cause other steps depending on the file field to fail.
To get around this, you can set the "Include file contents?" field to NO to exclude the file contents, and use the "Direct Media URL" field provided by Dropbox instead of the "File" field: