If the only KM Variable you are having problems with is %DropBoxLink%
then I'm pretty sure the issue is due to how DropBox works.
When you click on the menu item in Finder to create & paste the DropBox public link to the Clipboard, it can take up to 3+ seconds before the Clipboard is updated. So you nee a Pause Until action the Clipboard has been updated. For an example, see:
BTW, there are better methods to get the Dropbox link.
I'll post back here in a bit when I have found mine.