Ability to undo changes from outside Vim

In my vimrc I have

  set undofile

  set undodir=~/.vimundo

-- thus I am almost always able to undo changes done in Vim even if I close a file and reopen it. One other benefit I've enjoyed for a while is the behavior where, if I have a file open in Vim, and some program besides Vim modifies the file, not only does Vim prompt me about whether I want to load the up-to-date version of the file, but even if I do so I can still press u to undo the *external* changes. However, today I noticed this isn't working anymore on my Windows machine (I will check on my Mac and Linux later).

Does anyone know how I can make it behave the old way again?

--
--
You received this message from the "vim_use" maillist.
Do not top-post! Type your reply below the text you are replying to.
For more information, visit http://www.vim.org/maillist.php

---
You received this message because you are subscribed to the Google Groups "vim_use" group.
To unsubscribe from this group and stop receiving emails from it, send an email to vim_use+unsubscribe@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.