If you look into the Notifybox.txt file on the computer that has checked out the file - can you there see that the file checked out is listed?
I.e. is there a row with the file name, computer, username etc?
If yes - is this Notifybox.txt file syncronized to all other computers at that moment in time?
If you have a very slow Internet connection it might take Dropbox a few seconds to syncronize the updated Notifybox.txt file to the other computers.