Automator and AppleScript Login Items Disappear

I'm trying to get Dropbox to launch after a delay so I have time to type in my password to mount my Data hard drive whiuch is encrypted. I've written both an Automator app and then a AppleScript app that wait 60 seconds and then launch Dropbox. They both work perfectly when triggered manually but when I add them to my Login Items in the Users & Groups pane in System Preferences they don't trigger on login. When I go back to the Login Items pane they aren't there anymore!

I have several other apps and things set to run at Login and they all stick and work perfectly.

Any ideas?