Try reinstalling Dropzone. Steps below:
1) Close Dropzone by clicking the menu item and clicking the 'Quit' button.
2) Open the Terminal application located in /Applications/Utilities
3) Copy and paste this command: rm -rf ~/Library/Application\ Support/Dropzone
4) And this command: rm -f ~/Library/Preferences/com.aptonic.Dropzone.plist
5) Goto your Applications folder and drag Dropzone to the trash.
6) Delete any zip files with Dropzone in it from your Downloads folder.
7) Reboot your Mac.
8) Now reinstall Dropzone either by clicking Install from the Purchases tab of the Mac App Store or by downloading a fresh copy from http://aptonic.com/dropzone/latest
Hope that helps, but if not open Console.app (it's in Applications/Utilities) and do a search for Dropzone and paste me any messages. Maybe there will be a clue in there that will help.