My company uses the cloud service Box to deliver reports. I set up a Watch Folder to upload those reports to my Evernote. It works fine on first creation, but it does not upload subsequent additions to the folder. I've set the Box folder to actually download the files and not just link to the cloud copy, but that didn't help. If I delete and recreate the Watch Folder, it uploads all of the files and then again doesn't upload any files added later.
I've tested on my own Box folder and if I upload a file to the Box website and let it sync down, it doesn't get imported. The file dates and folder dates update. If I drag a file into the sync folder, it does sync to Evernote.